Yeah...
You likely can't make such a tight bend with 3/8"... perhaps with 1/4"??
Two tricks I know of:
1)freeze water in the line before the bend (not so good for this round and round thing)
2)put sand in the line before bending - fine sand here.
You really just need to make up a "slip roll" type of affair. Three rollers, with a round groove in at least one to guide the tubing. that and a crank... feed it in, it comes out bent... the distances between the rollers determine the circumference of the bend...
Good news... Harbor Freight sells one that mounts on the edge of a bench for ~50bux.
Trick three... run an asphalt roller or equivalent over the tubng - it's now flat ribbon, wind that.
Trick Four - perhaps the right amount of
HEAT[/b] applied as it is being bent over a solid (pipe?) form might work... I'm thinking of a pipe on two bearings/pillow blocks/lathe, with a follower roller affixed just before the tubing reaches the pipe, the pipe being rotated dead slow under power, and HEAT being applied as it reaches the bend.... but the Harbor Freight tool is easier and faster.
Trick 5 - you can use/make a "bender" that is merely a pipe, with a way to clamp the tubing and an arm with a roller that runs around the pipe for like 1/2 of the circumference, then you'd have to index the work in half turns as you bend it... but the supply would have to move with the bend or you'd reverse kink it... if you turned the pipe, roller fixed - that's a variation on Trick 4
I'd think that some copper flashing, sheared into strips would be more than sufficient... flat being easier to work with??
_-_-WBear2GCR
PS. oh, another way to go is to make a much bigger, loose coil, like say 6-12" diameter - then affix one end on the form and rotate it around until it pulls down to a smaller size - but again if it's really 3/8" OD, then a 3" diameter is probably too tight.