I use a 40watt high / low temp hot glue gun that was laying around the house. I think my mother bought it years ago. It doesn't always work the best, but it is old and I got it for free.
I always use the "high" setting. It can melt the foam a bit, but you have plenty of time to lay out long beads and glue up some wings or square tube fuselages.
Definitely go with the 40watt, but anything higher is too hot.