Modular pits

I'm recreating a published module map and there's a 30' long, 10' wide pit that is a garbage pit.
Currently I would need to modify that in Photoshop but I'm putting it on Roll20 and I don't think we can modify after export (can we?) for Roll20 so I'm kinda stuck.
I'm requesting a modular group of pit 'pieces' to allow us to create larger pits falling into darkness.
Something simple with corners, side pieces and a black 'spanner' to fill blank space.