You can download the attached STL file for holding the legs in place while soldering everything in place. This makes it all A LOT easier. Note that the tollerances on this are quite tight and you may need to scale the jig a few percent either way to make it fit your LEDs.

It's designed for 12mm APA106 leds, but you can scale it as far as you like. If you want to make a seven seg out of 3mm leds, you should be able to scale this all the way down.