Thanks, CB750.
The sheet I used is actually 0.090" thick black polystyrene; roughly the thickness of two pennies, stacked. The part came out lightweight, durable (with flanges on the tank-side, rear, and partial wrap around the sides of the frame), and I think thick enough to withstand the heat of the electrical stuffs. The next size up would have probably worked too (0.118") but I'm satisfied with this thickness and material. ABS plastic was my primary choice but not worth the extra skrilla.