maybe fabricate some out of plastic screening, plastic rebar and styrocrete or aragocrete etc
a little bag of cement makes for a lot of fun actually. throw everything in it but the kitchen sink(unless it is plastic, then chop that up and throw it in too

)
youcan sandcast big flat plates, and the cast side looks like whatever it picked up from the cast material, which could be even small rubble w/e. it is cheap enough to try all kinds of stuff until you are liking the results.
edit: styrocrete = styrene-crete, NOT styrofoam-crete, better make that distinction. i dont think i want styrofoam in the water. i'm thinking like modelling sprues etc