If the frame is a one-off, an inexpensive jig can be made of 3/4" plywood and V blocks. I believe this jig was originally posted by Glenn Thompson of Daltex Custom Bicycles in Dallas years before he became a professional framebuilder. I saved the photos years ago from a post of his thinking I might try using something like this (long before buying my Bringheli jig). You have to be careful with the torch; plywood burns.