Good question... I know Lowes/HD has plexi, but not the softer ABS. Maybe an arts & crafts store or Hobby Lobby? Or, pirate it off something you find at a second hand store.
I used black artist canvass. Lighter than plastic. It's been rigid and working great on my bike for more than a year now. But it doesn't rain much here in L.A. so I don't have to worry much about moisture.
I'm assuming to make stiffening plate for the inside of the swingarm bag. Cut it to fit the back panel profile, slots for your straps or holes for your brackets. Prevents drooping. Nobody likes saggy...
I'm thinking of doing the same thing with a sheet of 11g T-6 aluminum, powdered so it doesn't oxidize ('cause it's what I have at hand, fo' free) when I put a bag on my scoot.
Same here, aluminum plate. Works great and it is a nice hard mount that is invisible once installed. I painted mine black to match the bike and side bag.