The manual calls for the drive washer to be 4 inches from the firewall. If the cowl is long enough, you could possibly move the engine forward on the mount.

On one model my brother made a bracket to mount the receiver battery to the bottom of the engine mount and extended the wires to reach the switch. This resulted in very little "added" weight.

My 40 size Corsair has a very short nose. It needed 8 ounces of lead bolted to the engine mount. It flies very well.

As far as attaching weights, the method you used is as good as any. Attaching it to the very front of the engine mount maximizes its effectiveness.

Carrell