I've actually been thinking of something similar, but couldn't figure out what to use to keep the filler material from getting pushed down into the cart. The spring isn't a bad idea at all, though I'm not sure that the shape or material is ideal.
Ideally I think a piece of extremely coarse/porous filter material would be ideal to fill the majority of the cart - especially if it could be permanently bonded to the thin layer of filter that would touch the bridge.
Something like:
I have a suggestion for this, think it sounds possible?:
For the course material possibly use the fish tank sleeves then the normal cart filler on top, and spin a sring down through them both to get them attached. I am suggesting the blue foam as unless you cut the foam larger than the cart it is very porous, and does not stop a leak untill compressed further by using a larger section.
Just add spring in the middle of the pic above for finished product.
Spining the spring through both before insertion is probably the best way, then insert the completed assemly into the cart.
Glue wouldnt be a good idea lol, as bond material/method needs to be porous, hence the idea of using a spring. Another aproach might be to use some sort of mesh or bar on the top to hold everything in, but I cant think of an ideal mesh (or method to get mesh to stay in), and the bar idea would either need to be glued in place (not good) or have micro holes drilled in the side of cart ( more work than I feel this warrants lol) to slide the bar(s) through.