Well first off, fleece shouldn't absorb, it should wick, or soak through, to a layer that will absorb, like towels or mattress pads.
Secondly, if you don't have coroplast you can use any number of things. The most popular alternative choice in linoleum. However it is rather difficult to work with. For the next suggestion you would need to have the cage sitting directly on the table (or floor) without a grid floor. Just drape some plastic over the table or on the floor and put the grid walls on top. You can either use trash bags, or get that plastic from the paint department that painters use for a drop cloth. I have even found small rolls or sheets at the Dollar Tree down the street. You can even lay out your towels/mattress pads and fleece and then place the grids walls on top of that. The only problem with this solution is you have nothing to keep poos from falling out the sides.
You can even use a vinyl backed table cloth, or a vinyl table cloth or one of those cheap plastic picnic table cloths. Anything you can find that will not allow water through can be used.
Put down your waterproof layer, your absorbent layer and your fleece layer.
Hope this helps.