3/8" can bend pretty tight. A cheap and easy way of not crimping it for tight bends is to braze a cap on one end ,fill with sand or water, then seal the other end. For water-filled, get a flared fitting with a valve or find a bolt that goes in tight. For sand-filled, just braze on a cap on the other end. When done, cut off the end pieces.
I have rolled 3" diameter with 3/8" tubing no problem. For the size you are dealing with, an 8" coil would be plenty.
You could do a double coil by using a Campbell's soup can (still full) as your inner coil and get a large diameter PVC pipe fitting from Lowe's for the outside diameter. inner coil goes top to bottom and outer coil goes bottom to top. pretty easy actually.