I also needed one of these 1/4-20 threaded adapters. I decided to "volunteer" this angle bit to donate it's threaded adapter. The drill bit is a heavy press fit into this adapter, so I made some little pressing tools out of some hinge pin in order to progressively press the bit out of the adapter.

The main body of this custom right angle drill needs to be made of two pieces of material, one of which is 3/16" thick and the other 1/8" thick. This first piece is from a scrap horizontal stabilizer attachment bar.

...and the 1/8" thick piece is from a scrap popup-tipup arm.

The input shaft needs to lie on the split between these two pieces. So, I used a die grinder with a cutting disc to create a starter groove in each piece.

Then drilled through. The starter groove did a reasonable job of keeping the drill centered.