getting aspen to pack down

I recently switched to aspen and really like it. I noticed that in my tubs made of polyethylene, which are harder and sturdier and doesn't have a completely smooth bottom, that the aspen packs down to form a mat. I like this.

However, in my polypropylene tubs, the bottom is smooth and more flexible, and instead of packing down the pythons push the substrate around, into water bowls, to on side and pee on the other etc. Any way around this?

I have contemplated putting a layer of corrugated wrap beneath the aspen, but that negates the ability to spot clean if pee soaks down.