I built one and had no problems flying it; the plane flew very smoothly. I only crashed it because I had a loose battery plug and lost power on approach, causing it to nosedive, break the prop, and rip the landing gear and bottom half of the plane off.
I was running a 2200 mAh 3S battery in it, and pulled the battery just a little forward to make it a slight bit nose heavy rather than perfectly centered, and it flew wonderfully. As for mounting, I went to Hobby Lobby and, with a 40% off coupon, bought a pack of the Heavy Duty Velcro that was normally something like $20 and brought it down to a really reasonable price. I still have that original package, and have been using it to hold the battery in with no issues of sliding out at all. If you're still worried about it, you can glue some popsicle sticks on the underside of the plane, put the velcro in, and on either side of the popsicle sticks, make some slits and run a battery strap in and around it to keep the battery from moving. The popsicle sticks add stiffness to the foam, so it won't collapse when you cinch down the strap.