Nice ramps you made there.
To help keep them from sliding with the Polyurethane finish, you could glue some sandpaper or another abrasive material to the bottom of the ramps.
This might help to keep them from sliding around, especially on carpet or a slick surface. I would not trust putting rubber mats under these ramps, since any debris that is trapped under the ramps, might allow the ramps to slide on the surface.
To help keep them from sliding with the Polyurethane finish, you could glue some sandpaper or another abrasive material to the bottom of the ramps.
This might help to keep them from sliding around, especially on carpet or a slick surface. I would not trust putting rubber mats under these ramps, since any debris that is trapped under the ramps, might allow the ramps to slide on the surface.