You have a couple of options here

The easiest is to get some stick on lead weights and some fiberglass reinforced packing tape.

Tape the weight out at the tip of the light wing on the underside.

The other option is to cut the covering, again on the underside of the light wing, and glue with epoxy the weight to the inside of the wingtip and reseal the covering.

To find out how much you need, get 2 people and look at the balance like you already have done and add weight to the light wing AT THE TIP. Keep adding weight until it balanced then remove about 1/8oz to compensate for the weight of the tape. The more tape you use, the more lead weight you remove. Same with glue

The key is to add weight to the tip, not near or at the fuselage. In close to the fuselage will require allot more weight