whether for just one time or for repeated use, jigs and fixtures are always worth the time and effort to build, simply for the accuracy and being sure your operation goes the way it is supposed to go. to me it's just plain stupid to trust any sort of makeshift set up when a fixture can be had to ensure good results. if you do take the time and effort to build a good fixture, even if for 1 use, you will find that it will be used again and again simply because you have the convenience of doing the operation with the precision and ease of difficult procedures that a fixture affords.
some of the guys here are posting excuses to not build a fixture,...these are guys who never tried doing the operation with a fixture. if they would have, the only excuses you would see here, would be excuses to build more fixtures for building these guns.