I think the foam it's a good solution. I've also bought some plastic spacers with adhesive back: those things are specifically designed to mount circuits. I've found it on eBay, from a local electronic supplies store. Here's a pic of it:
CLICK HERE! In the image you can see three kind of spacers: the first from the bottom it's the one I've talked about, and it have a snap-on clip you can also open to remove the circuit if needed; the little cylinders are normal spacers with plastic screw in both ends; the square ones are designed to fix something (circuit or other pieces) with a zip-tie, and also have sticker on back.
Hope this can help someone.