I mainly use 10mm 4 wire LED strips, and while there are 8mm and 12mm soldering jigs out there, I present to you my vastly over-engineered 10mm soldering jig! :) Seriously, needed one for the 10mm I use and since I couldn't find one here, I made one.
Note: I use 22-gauge wire in this.
While it should work with just about any 10mm LED, I use https://amzn.to/3oHOXkx
Since publishing this model, I've uploaded the following for soldering two 10mm LED's together.
10mm 2x LED Joining LED Soldering Jigs by Jeff Smith - MakerWorld