Fill the gap with wood and it will be fine. Just don't take the short way around and try to fill with glue. You must fill the gap with wood and then the fillets will cover it. It will be strong just not as light as a tight fit.

I've stuck the balsa in when needed. Soft balsa works. I've got some crushable 1/16" that usually fits, along with wider stock, now and then. Jammed in, then I then run in thin CA, building up the space. Finish is 30 minute epoxy spread throughout the outside of the joint, when hit with a heat gun it runs in and fill all gaps. Careful not to put the heat too close to the wood. It'll bubble dope and silkspan. Do one side at a time to keep track of the runs. Have isopropyl alcohol and paper towels ready. Wipe off all excess epoxy. The final joint usually looks almost perfect. Trick is to not damage surrounding dope (I finish my fuses pretty much before sliding in the wing) and wipe off before the epoxy hardens. The heat gun will speed the epoxy cure.