Cut a narrow slot in the bottom of the fuselage near the tail.....wide enough so that shotgun pellets (the smaller the better...size 7 or 8....will fit into it.
Make it reasonably deep...get the pellets at a gun shop or have a friend cut open a shotgun shell for you....this is dangerous but I have done it.
Mix up a slurry of epoxy and pellets and fill in the slot.....works for me.....hope this helps.