Baltic birch ply is nice stuff for lot of things. I have used the 1/8" 3 ply stuff for some applications and think it would be adequate for this purpose and suspect it is probably lighter than the 1/4" corroplast folks would likely be able to scrounge up. Not sure you can find Baltic birch in very small sheets or not. I use the 1/8" stuff often enough to buy full sheets (which oddly enough are 5' x 5')