Have just used aubiose for some of Neptrs and pretty much all of the current fosters cage. I found even hay isn't good at making it hold burrows well, but it was really ideal when I had multiple rescues at any given time because of its price. Definitely something I will go back to if rescuing picks up again (right now I am content with 3 hamsters, so until the one foster goes, definitely going to slow down on taking hamsters in).
I definitely will not be rescuing/rehoming guinea pigs in the near future. It was more of a "look what we found, you like animals, here you go!" type of thing. They were rehomed to someone on the guinea pig forum, so I do get to see loads of photos of them in their own room whenever I go and check
Thanks for the info on silica. I think in my head I had it gathered like those silica gels that come in food that say "do not eat" lol. Definitely didn't know much about silica, so that was really helpful. I'll pick up a bag of that desert sand as it is something I've been meaning to try and I am getting a Savic anyway I think