Aspen is good for burrowing but i would not use it for a Coxi. it dries out very quickly and will not be able to hold the proper humidity unless u live in a naturally high humid climate.
Edit:
Originally Posted by asplundii
I have a few species that are prone to burrowing and I will concur with the coir-type substrate with a small caveat; if you can find some, then for every 5 parts coir I suggest adding one part high quality sphagnum (NZ or Orchid I find are best) and one to two parts oak leaf litter (you could use magnolia as well but you will need to break them up as they are much larger than oak leaves). I have found that using this mix helps keep the coir from compacting, helps the coir maintain a bit more stable hydration level (e.g., not drying out to a brick and then turning into a soggy mess), and creates an easier burrowing substrate for the animals because there are little air gaps and roughage for traction/leverage. I also think it looks a bit nicer/more natural that just straight coir.
is oak leaf litter full leaves or is it chopped/shredded or mulched?