I think the cheapest way would be to go with wood again, and take all your hardware from your old boom. Thats what I would do. Pretty simple too
Precisely !!!!!! find a lumber yard who deals in PRIME GRADE wood 2x4's, select a straight grain, 'clear - with NO knots - 2x4X12 ft. long. - go through the 'pile' and choose only perfectly STRAIGHT wood and 'clear' with NO KNOTS - eyeball it on all '4 corners' for straightness.
The foot length dimension of a M16 main is 10,33 ft., the MC is 11 ft. .... so a 12ft. CLEAR GRAIN fir or hem-fir 2x4x12 ft. (about $8.00+ for 'clear grain' , no KNOTS) would be your cheapest way.
BTW - You're NOT going to find PRIME Grade 2x4x12 ft. in a Home Depot or Lowes or other 'big box' stores.
No need to bore-in a boltrope track, just fly the mainsail 'loose footed' - no track slugs, no boom track. Carefully PRIME + OIL BASED enamel, etc. ...(many
coats, especially covering the 'end grain' at the butt ends to retard moisture uptake to prevent warping, etc. Use all the hardware you already have on the rotted boom ... just carefully 'bed it in' to the new 2x4