So I took a look at this website a papersource craft and was inspired to make my own.
Here’s theirs:

I haven’t taken pics yet but I took a small priority mailer, cut the sealed end off, made 2 scor marks in the center 10 cm from the edge and the trimmed excess 10 more cm from the scores and then trimmed a flap, slipped in 2 hedgies and away we go. Simple, can be reproduced easily with better materials and is strong. It hold 2 hedgies in a variety of ways and with a couple of velcro dots would be wonderful. I’ll get some pics of mine up a little later.

What can you make with an old envelope or 2?