I actually use an enclosure that has holes in it. Either the enclosure the PCB is meant for, or another that has a hole that works.
Put the part in the hole so that the part is on the outside. Put the PCB on the part (Switch, pot, whatever), and hold it in place with your fingers until you get the soldering iron ready. Solder it in, then hold the PCB with your fingers until the solder has cooled enough that it won't move.
Makes it easier if you have multiple parts in place. For instance, with the testing rig, if you pre drill the enclosure first, you can just mount all the switches in reverse order on the outside of the enclosure. Get the PCB on there, and even soldering one lug the PCB will stay in place and you can just whip through soldering the rest on there.
Jacob