The main thing is wanting something soft that doesn’t slip around. A lot of owners will put a towel, fleece, blankets, etc. directly on top of slippery floors, so it is not helpful for traction/padding, in addition to making it an easy target for digging up and chewing. A couple options come to mind:

1. A larger rug that covers a big area so it’s not easy to move around and you can block off some of the edges as needed (though usually when a rug is large enough, rabbits will be more entertained by whats on the rug surface instead of the edges).
2. Cotton throw rugs that are generally fine for ingestion, can be washed in the washing machine, and is also soft and can be put in different areas, depending on the senior rabbit’s preferences.
3. Fleece on top of somewhat firm memory foam— the fleece naturally clings onto the foam, so it is not easy to dig up: https://youtu.be/IzXXmQmgCFg
   1. I bought large rolls of foam and large rolls of fleece to cover some area for Wick for one of his past enclosure home-bases

For my senior I used sheepette, a medical type of faux shearling, on top of washable pee pads (which was a past suggestion from Wick&Fable!).  It gives good traction and any pee goes straight through to the pee pads so bun stays dry. I got a couple large pieces and was and rotate once a week or so (more often towards the end of his life when his incontinence got worse). I liked that it gave him much better traction than fleece or flannel.

They do chew it a bit… but my buns tend to rip it up and not eat it so I don’t worry too much about it.