The safest way to keep a flying squirrel, while keeping the flying squirrel happy and safe, is to either dedicate a room to your flyer, and "squirrel proof" it, or to provide a large cage. Purchased cages can be very expensive, but with very little talent, and a reasonable amount of skill, you can built a wonderful cage for a flying squirrel from 2 x 4 stud lumber, plywood, and hardware cloth.
You will need:
6 8' 2"x4" studs
2 6' 2"x4" studs
1 4' x 8' 1/2" or 3/4" plywood
20+' 1/2"x1/2" hardware cloth
1/2" by 1" wire can be used,
in place of the hardware cloth.
hand or power saw
(compound miter saw is perfect!)
4" drywall screws
two door hinges & screws
two barrel bolts
Cut 4 of the 2x4's into pieces 2' and 6' in length, and 2 into pieces 4' in length. This will give you 4 pieces of each length.
Make a mitered* frame from the 2' and 6' pieces, using the drill to pre-drill holes for the screws. This both makes the screws easier to drive, and keeps the screws from splitting the 2 x 4's. Lay the frames on their 6' sides, and attach the 4' pieces of 2" x 4" to each of the frames, at the corners, to make a box frame. Cut the plywood into 2 2' pieces, 1 3' piece, and 2 6" pieces, all 4' wide. Attach one 2' piece of plywood to form a floor for the box. Cut the 6' 2" x 4" studs into pieces 2' and 4' in length. Attach these as spreaders at the three foot level on the box, as shown in the illustration. Cut the hardware cloth into two pieces, one 8+' long, the remainder 12+' long. With a staple gun, railroad the 12' long piece of hardware cloth to the top half of the box, from inside the box. Attach the 8' piece in the same way, leaving one 4' x 3' opening, which is where you will be attaching a drop type hinged door.
With the two 6" x 2'pieces of plywood, you can make two interior shelves. Hang one just at the level of the nest box - or perhaps even a bit higher, say about 3" below the nest box opening. This will make it easier for your female squirrel to access the cage, with a pup in tow, should you have a breeding pair. Hang the other on the opposite side, at a different level. You can either use staples to attach the shelf directly to the wire, or you can buy wood 1/2" square moulding, and slide 3 2' pieces through the wire, level on both sides, to support the shelves (two on the bottom, and a third, on top, to keep the shelf from slipping). If you use the staples, you will of course need to use a step ladder, to support the shelf on the inside of the cage, while you staple it from the outside, through the wire, to the shelf. If you have a youngster, you might be able to enlist their assistance, in holding the shelf while you staple - but don't forget to let the child back out of the ... wait. Now that I think of it, this might be a useful cage for ...well, nah. Never mind ...
Attach a 2' x 4' piece of plywood to the top of the box. With the two hinges, attach the 3' piece of plywood to the 4' 2"x4" to close the opening. Attach the barrel bolts to the 3' piece of plywood at each side, toward the top. If desired, you can attach stop chains to each side of the drop door, so that it can be left open for your flyer, in the evening, while they take some flight time exercise. You can also attach a pull handle, if pulling the door open by the barrel locks is awkward, for you.
Attach casters to the bottom of the cage floor. Buy or make a flip top bird nesting box, at least 6"wide x 8"long x 10" tall, and drill a 2 1/8" diameter hole on the long side, closing with hardware cloth the hole on the short side. Attach two screws, at a slant, to act as pegs on which to hang the nest box to the top spreader of either 2' wide side, and drill holes in the top of the long side, through which to hang the box on the screws. Cut a hole in the hardware cloth to match the nest box hole opening. Be sure to use pliers to turn back the raw edges of the wire. Secure the box to the hardware cloth with a bungie cord, to keep it snug against the wire. The flip top can either be kept closed by simply placing a brick on the top, or a heavy stone. If preferred, you can put an eye and hook type fastener on the nest box lid.
Voila. You now have a flyer cage that offers the flyer safety, and offers you mobility. You will be able to take the cage from room to room, as it will easily fit through doorways of most standard size. If you have doorways of less than 2' width, adjust your cage sides accordingly. The cage can be outfitted with "playground apparatus" consisting of simple light wood branches (such as willow), and climbing ropes, etc. Your flyer should only be fed in the cage, and water should only be provided in the cage. Food dishes can be put on the floor, as can water, or hung on the sides, as it suits you. Extra nest boxes can be hung in the same fashion as the first. This cage will accomodate two flyers, perhaps a third, if you provide at least one more nest box than squirrels. A pair works better, IMHO, than a threesome.
(*The drawing does not show a mitered frame construction. Squared frame construction can be made, but the ends of the 2"x4" will need to be cut for notch/lock-type assembly (like Lincoln logs), or you will need to counter sink the screws that run through the 4" depth of the 2"x4"'s)