A large indoor aviary makes a good Flying Squirrel habitat. This needs to be designed for smaller birds such as finches, as otherwise the bar spacing may be too large and allow escapees. Arboreal vivariums are another option, although it is important to add additional ventilation to these. Wooden garden trellis make excellent climbing opportunities to add inside your vivarium to make up for the lack of climbable bars.
Although slightly smaller than sugar gliders, the minimum cage size would seem to match activity level - with 4 foot high, 2 x 2 giving a good starting point.
Furnishing the cage can involve items such as hanging nests and hammocks, wooden and rope parrot toys, and sisal rope. It is important to ensure your squirrel still has a decent area for jumping and gliding so the cage should not be too crowded. Multiple nest boxes are a good idea, especially if you have more than one animal.
A good idea for a home made cage is to have both a full hinged door to allow cleanouts, but also a much smaller door to allow quick food and water changes.
Further ideas on this can be found in our furnishing a vivarium for an exotic rodent article.