Hi Kevin,
I have installed quite a few foam seats. I would recommend 3M spray on contact cement. If I remember correctly it says Super 77 on the can. GOOD STUFF!
I would not recommend velcro. It doesn't stick to minicell very long before the water causes it to get soggy and peel right off. As far as making it fit tight. My last seat back made of minicell not glued in, blew off on the highway as well as the cockpit cover I thought would hold everything in. Make it fit tight play with it to get it in the right position then glue it in.
Kent