Hey there, I did this in my old Ferplast 160. I used a wire shelf that you would find in a greenhouse, cable ties, plastic guttering (square type) and carpet.
Attached the wire to the cage sides at the desired height. Used cardboard as the flooring And then fleece but that part is up to you. I made up the cardboard came up the sides To prevent pee spraying up the walls, and ensuring the wire parts were sharp or within reach. Drilled holes in the guttering and attached to wire flooring with cable ties, double sided carpet tape to secure the flooring and making sure the ramp end of the guttering was tapered.
My pigs loved it. I probably would use better flooring if I was to do it again.