i have made about 5 different lexan side panels for my coolermater 201 case over the years, i use 8 thumbscrews to attach them to the case after i drilled and tapped the holes for the screws.
everytime i get a new system or heatsink i make a new one for ducts that i make to bring fresh air into the heatsink.
use 1/8" lexan, not the cheaper plexi, much easier to cut and way more durable, worth the extra few dollars IMO.
i use a table saw, jigsaw and a dremel for the cuts.
here is a pic i took yesterday of a new panel in progress, the green tint is still on the one side, i am still working on the duct on the rear that blows the exhaust air up instead of towards the wall, the duct to the BT heatsink and need to finish the cpu fan wireing, it is just hanging there for now.