Need help with my guinea-pig sleeping bag please!

[Alicia]

I don't know if some of you will remember my last attempt at making a guinea-pig bed (it resulted in Roy sitting in it and it all fell apart around him ) but I've decided to try again. I've sewed together the main thing (not very well but I think it's better than the last attempt) And now I need something to keep the opening bit open. It collapses in on itself so the guinea-pigs can't get in. I've made it for Roy because he looked a bit girly sitting in his pink bed and he wasn't impressed so now he has a manly bed.
How can I keep the openings open and is there anything I can improve on it:

Inside:

 

[keli]

I've made a few too and I find this to be a problem.
I've tried using a wire hanger and stiffening. As far as I know some people use wadding or you could fold the top down a little bit and that kinda stiffens it up, not much though

 

[flips]

You could fold the end over a couple of times to make a cuff and stitch it in place. That should help. Looks great btw. Great effort and I'm sure Roy will appreciate it.