I have a strip of Baltic Birch Ply in the Bottom may put a couple screws thru the bottom of the bag into It to stop any sagging at all ..

Had A Karrimor saddle bag for years they sewed a pocket in the Bottom to slip a stiffener into, the bag shipped with one in cardboard ,



I, again, got a thin piece of Baltic birch Ply (the plys of all birch are thinner than Birch faced Fir Ply. 1/4" its 7 ply)


Coroplast (Corrugated Plastic) is made in LDPE, the common stuff , and Polycarbonate (lexan) much stiffer .. big cities should have plastics specialty stores

or ask a Sign Shop.