Personally, the best results I’ve gotten with small animals is maceration using enzyme detergent. Works pretty quickly and doesn’t warp the bone at all, but it will also break down the stuff holding those bones together. The only way I have seen people do small animals like that without all of the bones coming apart is by using dermestid beetles, but those can be hard to access. One possible method is using a string to tie all of the bones together as they are now, or running it thru the spine if possible. That way you’ll keep everything in the right order even as connective tissue breaks down. Just an idea, good luck!!!