Simple soldering jig

Step 2 Foam is the magic ingredient

Get some spongy foam that's a bit bigger than your tin. Start with a piece much bigger than your tin and reduce it until you get something you're happy with. The more foam you use the more pressure will be on your components and PCB and the less they will move around. But also the more the lid will want to pop open, so keep this in mind or fasten the lid with tape.

DANGER! Soldering can get very hot, so choose a foam that is flame retardant. It's better to play it safe than to explain to people how you burned your house down.