Hi Paul,
Usually the tips pn fpam wings are carved and hollowed. Follow the plan view for the outline and then carve the shape to the airfoil at the tip. Ronud and shape to suit. After you have sheeted the wing, use two blocks which add up to the thickness you need, tack glue them together, cut out the top view outline and tack them onto the wing. Wrap a couple layers of masking tape around the end of the wing to protect it while you do the rough carving and sanding. Then remove the tape and do the final sanding, carefully. Pop them off and hollow. You can build your tip weight box right in and make provisions for he lead out guide.