Rather than carve wood, use soft wax, from children's crayons, or the wax coverings from posh cheese, or even mini baby-bell.
Wash the wax, then float on top of hot water in a bowl, it will form a skin of wax, fish out with a cocktail stick & mould with hands, & press into nose cavity, previously lined with foil or cling-film, skim to edge with a ruler & repeat for other half.
Practice with simple shape like a drop tank to make sure you can get it out in one piece, popping it in the freezer may help...
When you have 2 halves you have masters to use plaster to make moulds.
Any plane I've used nose weights on i have also drilled a pin through the landing nose wheel & strut.
Another option, a company SAC(?) Make nose weights and metal landing gear already...
http://www.scaleaircraftconversions.com/homepage.cfm