I wanted to get a profile that was as short as possible for CQB.
This is not threaded because spinning it would be impossible, I use slim tape against the threads to press-fit this. You can also use anything that clamps onto a rail to hold the joint.
This was made to fit my 10yr old Dboys Scar. I have no clue if it will git other models but I included the source file for editing.
The picatinny source is from the Sketchup Warehouse.