Petarus brevicapus fan
I agree with BYK_Chainsaw, but I'll add one thing; Make a pouch out of one of your raggy, old pieces of clothing, and make sure it still smells of you if you have'nt used it in a while. Ta-da! A cozy place that smells like you! Just make sure it's sturdy and safe. If they do not want to use it, try posisioning it in their favourite part of the cage.

BYK_Chainsaw
Time, patience, MORE time, consistently spend time with him. and TREATS are great. Take baby steps if needed, look for small improvements. talk to him, I would sing to one, he would stop crabbing and just look at me. and since you are huge human and they small marcupial, give him Time, even MONTHS to adjust to you if needed. some glider are fast to like you and some are slow, very slow, but you keep trying. Use Dark room at night. TREATS., mealworms, yogurt drops.