I agree, lead fishing weights and Plasticine or Blue Tac to hold them (Don't need superglue - mold the Plasticine/Blue Tac around the lead, and press into the void).
There's a lot of room in the nose void, in relation to the size of the fuselage and C of G, so cram in as much as possible - better to have a lot than not enough.
As a test, before continuing construction, add the weight, tape the fuselage halves together, and slot the wing in place, taping it if needed, to hold it in place. Then, balance the model by the wing tips, on your fingertips. If the nose drops, fine - if it doesn't, add more weight until it does!