Personally I would forget drilling the spar hole just right. Drill it quite a bit oversize to make plenty of clearance for the spar. Then just butt your clamped blocks to the end of the flaperon, carefully lining up the leading and trailing edges with the split in the blocks. Then just trace around the flaperon end with a marker and your ready to bandsaw out the profile. Use the outboard end of the flaperon and you don't even need to drill a clearance hole for the spar.