Ajja has done this twice, and I know for a fact she's using the slot at the bottom of the background. She's 3ft+ and about 1500 grams. If she can get back there...
You REALLY don't want to leave your BP back there. I came home a few weeks ago to find Ajja back there. I don't know how long she had been there, but she was FREEZING. since then I've had stacking stones or the base of a fake plant wedged in there. I'd try that before gluing or Velcro. Both of those will be an absolute nightmare to clean, not to mention what a beast it's going to be if you ever have to deal with mites.
If you can't find stacking stones locally, LLL has them on their website.
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