the cage should be atleast like 65cm in all direktions and no more large then 2cm bar spasing. they are arboreal so alot if vertacle spase is good. natural wood bird perches are good for inside the cage, i recomend manzanita with bark on it is a very tough wood and easy to clean because its not porus. but also they need some flat perches like shelves(that are solid not wire). and for a nestbox something enklosed and near the top of the cage, like a birdhouse type thing or a cloth bag with a hole in the side. also i think they will use a large excersize weel. if you have alot of time and you would like to make a cage then it definatly would save money, but it may take alot of work so really its up to you. im not an expert of flying squirrels but i hope this helps a little.