Rash, you could just go down to the local sheet-metal supplier in your town and pick out a piece that's the right gauge. It's probably not worth the trouble modding the sidepanel from another, older case, although you might get lucky with that approach as well.
But the problem arises when you have to make folds or bends in the sheet-metal, and you don't have a "press-brake and slip-roll" machine to do it.
You could try this, though, but you'd better plan the work carefully or you're going to destroy that case side-panel.
Cut the panel so that the edge-fittings on all four sides are intact, and so that you now have a metal frame -- with maybe an inch and a half of sheet metal on all sides. You can then do (at least) one of the following things:
1) Cut the Lexan to fit the frame with edges flush with it, carefully countersink holes in the Lexan for quarter-inch-long 6-32 flat-head screws, and secure the Lexan that way. You would use a $5 6-32 screw tap and drill from Sears to tap the screw holes in the metal. In this approach, the Lexan panel will not sit flush with the remainder of the case metal, but it might still look good.
2) Very carefully cut a frame for the Lexan that just overlaps the main panel by at most an eighth of an inch. VERY CAREFULLY position the panel on this frame, and glue it CAREFULLY SO THAT THE GLUE DOESN"T RUN OR DISCOLOR THE PLASTIC -- all the way around. THEN -- this rectangular Lexan frame would fit on the inside of your panel, and the main Lexan window would be flush with the case-side-panel and fitting in the rectangular hole you would've made according to (1). You would trim the hidden Lexan frame so that it doesn't interfere with the sheet-metal fittings for your chassis. You can either then tape the hidden frame to the back of the case-side-panel (frame) with a special tape from Home Depot (see
www.mnpctech.com for a video on side-panel windows) or figure out another approach to using the 6-32 flathead screws (from the inside). Most likely, you'd still drill through the metal and the resulting tapped screw holes would go all the way through. But you could carefully trim the excess metal and smooth the surface so -- once painted -- you wouldn't see it.
My only reservation is adding screws to a chassis that could come lose and fall inside it, but between hard-disk screws, fan screws and motherboard standoffs, there is always a risk there anyway. Even so, you can dab a bit of "Automotive GOOP" adhesive on all case-interior screws to keep them from falling out and keep them from coming loose.