The model I working on has (open construction wing tips) meaning I can look straight through from the top.  The wing tips are quite thick and I am wondering if anyone has ideas for putting a weight box right into the wing tip.  I am thinking a tube (either aluminum or carbon fiber) with a cap or something of the sort.  Any ideas?   

I have seen tubes used for weight boxes and it works just fine.  Don't worry about keeping it light, though.  After all it is a weight box - brass, copper, whatever will work great.

This must be a scratch build as most kits show how to do weight boxes.  If you do a search and spend some time reading you would find not only the two ideas mentioned but many others.   if I sound harsh it may be I'm still recovering from an all night drive,   But even in all my years of modelling I still do a search on here as well as reading each day and still learn something new even from the kids that post on here..   

So except my apologies.

Tube type, for weights I cut various lengths of 5/16" steel rod, use a nylon set screw for cap.

Finished install.  Control CG with light spacers to position weights fore / aft in tube.