My brother wrapped his receiver battery pack in foam, then wrapped that in clear packing tape; and attached it to the engine mount (in front of the firewall) to avoid adding any weight to a Sky Raider Mach II. He drilled a hole in the firewall to run the battery wire to the switch.

If you do not want to do that:
You can drill and tap the bottom of the engine mount for a 4-40 screw (do it as far forward as possible). Drill a hole through your weights and screw them to the mount. As mscic wrote, the further forward you can move the weight the less weight you will need.