Charlie, this is a Nexstar 46 and if stock, it has a single central servo. So, in that case, I would either put the weights as Gary suggested slot and glue into the wing-tip balsa block if it has one. Otherwise, as Gary suggested, glue them in on the last rib then add a small piece of covering to mend the slot.

The OP can also use some of those self-adhesive lead weights that we put in the nose to balance the CG. Heck, for short term tests, those can be placed right on the clean covering on the underside of the light wing-tip and stick one or two right to the covering, fly it to make sure it's ok, then, once satisfied, put them directly on the balsa wood under the covering.

CGr.