Have you considered making the buckle out of styrene/ABS? Google and see if there's a plastics shop around (they sell stuff like sign sheeting and perspex), or you can hit up a hobby store. And get a bottle of Zap-A-Gap / CA glue / styrene glue. It'll give you a lot more rigidity to the buckle, and will look more buckle-like than craft foam.
As far as closing the belt goes, I'd do a double-snap (one snap above the other) hidden under the pouch.
The only other suggestion is that if you want to add velcro in there, tack it on to the back and as you sew the details, it'll sew the velcro on as well. Or, buy sticky velcro.