Just exploring the possibilities....
Could you use 2 or 3 inch round tube for the frt main keel and mast complete
with a bend in it so no holes except to seal up the ends and a supply hole and vent hole in the fuel supply cap?
maybe use 4130 unless 6061 t-6 round tube or square and be bent w/o it causing structural
issues.
Visual example in this Single Seat Arrow Copter clone drawing.