Cutting out IO ports in the back of case

I want to add 9 serial port holes to the back of one of my cases. They need to be done right because the internal serial port cables use screws to be mounted securely. One screwup with too big a hole and the serial ports won't mount correctly.

What tools do you guys recommend to do this? I have a dremel but I'm not so confident in my ability to carefully cut a perfectly shaped hole.

Would you use a dremel for this or something else?

Use a USB to serial convertor instead? Or just buy one of those brackets you linked?

There are lots of brackets available with two ports. But yeah, you would need five of them...
I'm not sure if three ports would fit on a single bracket and at the same time support how big some serial plugs are.
There are USB to serial converters with many serial ports, but they could be pricey.

If your aim is not strictly serial ports but to use DE-9 for vintage game controllers (8-bit Atari or Commodore joysticks, Sega, X68000, FMTowns etc.)
then maybe you could perhaps use brackets for 9-pin serial and 25-pin parallel and use a Y-cable from the parallel port to the game controllers. I know these existed for the Amiga, but could perhaps be hard to find ready-made these days although schematics should be available. That would reduce the number of brackets to three but would also require internal rewiring.

If you still want to do it yourself, then Dremel could still work. You could also use a drill press and router table. You will need a router bit for metal that will fit into your tool and into the radii of the corners. You will also need a flat needle file (but do get a set: they are useful.)
Do try to find brackets of aluminium that are much easier to cut than steel.
Print out the pattern, or transfer to graph paper. Glue the paper onto each bracket - I use a glue stick, or use self-adhesive paper. Drill holes for the bolts and in each corner of the port with the correct drill bit. If you are not confident in your ability to drill straight, drill first a smaller pilot hole, measure that it is in the correct position and lean into the hole to compensate for your drift if necessary.
Use a router bit for metal to remove most of the bulk inside each large trapezoidal hole, but don't do the edges. Finish the edges using flat needle files. If a corner in a trapezoid is somewhat off, it is also easier to make it larger with a round needle file than with a drill.

I'm not one for ghetto but replicating that shape could be hard. If you have watercooling hose ports on your case you could just run them all out through one of the ports then fasten the cables so they only hang out a few inches, then just plug them in. Or just run them all out of one of the pci bracket slots.

Why not just have a custom CNC milled replacement slot cover - 3 slots wide, 3 ports on each slot, with a single fold for mounting just as those single-slot covers do?

Custom CNC routing might be out of the price range.. Just saying. There are a few companies that specialize in making front panels for electronics hobbyists. There is also BigBlueSaw that has been used by the custom keyboard-building community for building key-switch mounting plates.

A drive slot cover could be more affordable if made from laser-cut acrylic, but to fit panel-mounted ports the plastic would have to be very thin so you would have to reinforce it by gluing plastic strips to the back as a support structure.

I would add the serial ports by having them mounted to a piece of thin hardwood. Then you can mess with the cutouts an not screw up your case practicing. Once you have the wood serial port holder built, hack a huge rectangular block of metal out of the case and mount the wood there.

This is what I would do:
1. Use a 0.7-1mm (or whatevet the imperial measurement as close to 1mm is as possible) sheet of mild steel.
2. Use sheet metal cutting shears (special scissors) to cut out a square sheet that would cover 3 slots (or whatever is the amount you need for all your ports). Could be a bit larger.
3. Cut out details, like the "fingers" that slide behind the motherboard and the fingers that you will later bend to mount the thing (it's easier to bend 3 "fingers" than a whole edge of the sheet). Use a dremel cutting disc or shears where it is more appropriate (I'd use shears where you can reach and only use a dremel for unreachable places, like the piece between "fingers).
4. Get a drill bit, whose diameter is the same or slightly larger than the heigth of your port. Use it to drill two holes, with each hole being where the end of each port would be (so a total of 18 holes for 9 ports). Or, you could use something close to a 2-3mm drill bit and drill holes where the corners of the ports will be (28 holes for 9 ports).
5. With the holes in place, use a dremel cutting disc to "connect" the holes and cut out a rough shape of the ports.
6. Use a file to finish up the fine details (inside corners for ports etc).
7. Drill all the holes for mounting.
8. With some pliers bend the "fingers" where needed. This is when you could shorten the bent fingers so that they would fit and drill the holes in them for mounting, if that will be easier for you.
9. Test assembly, disassemble, paint, assemble, mount.

I left out the details like marking where to drill, sanding the edges of the metal part so as not to cut yourself etc.

that's about the best way to do it!

Forgot to mention that it's a good idea to make a stencil out of paper or cardboard first.

Just get five of these and save the time, effort, and risk of getting hurt.

2 in 1 2-port DB9 RS232 9pin Male Com Port Serial Cable Cord Host Bracket PC

or see if this guy can work something on only three of these instead of 20.

3- Serial 9 pin RS232 Motherboard Ribbon Cable Slot Bracket - Lot of 20 pieces

I agree, three 3-port brackets would be the best solution. I had been looking for them but did not think they existed...