The simplest bicycle framebuilding jig I could come up with...

Step 5 Angle indicator / beam lock

The piece locks the beam at the correct angle, and works as an angle indicator. After you have set your chainstay length at the bottom bracket, you set the seat angle here. It should be self-explanatory, really.

It's a #4375 Inside Corner Bracket. I milled a slot connecting the outer two holes with a milling machine but you could drill a series of holes and then finish the slot with a round file. A jeweler's saw would work too.

The T-handle is a standard 3/8" model from my local hardware store. I used a short carriage bolt from the hardware store, cut to length and ground it to fit the slot. 8020 does sell T-slot Studs but they are 5/16" or 8mm, not 3/8".

I took the nerdy approach and mocked up the angle indicator in Autocad, printed it out, and used the paper pattern to file little notches in the end of the angle bracket. They line up with one of the 8020 Align-A-Grooves to indicate the angle. The groove is inked so you can see it better. It's probably faster to just set the notches by just using a protractor to set the angle between the adjustable beam and the frame.

The angle bracket attaches to the beam with yet another double T-nut and matching 16mm M8 button-head allen bolts.