This tube may work on your OTA project. It may flange up to Borg equipment, but I am not sure.
I recently upgraded (or down graded) my FS-78 (78MM F8) to a shorter tube to allow more infocusing for video photography. This is the original aluminum tube. It is beautifully baffled with flat black inside. Takahashi cream/white outside. To the best of my measuring ability (with a standard tape measure) it is 14-7/8" long, 3-3/4" OD, 3-9/16 ID with outer threads on each end. It has a small scratch in the paint (3/4" long by 1/16) on one side.
Will trade for a SCT to 2" visual back adapter.
Or $25 plus shipping (money order). Or pick up in Deer Park, Texas (next to Houston) and save shipping.