You might have to either:
Install an external (immersed) ground plate c/w thru-hull bolts (best solution). Gnd plate will be 144 square inches minimum, better a long narrow strip (48" or longer x 3" wide), and NOT sintered.
Utilize a much less effective temporary connection, such as a ground plate connected to a large (2/0) cable c/w clamp to attach to your mast stays or chain plates.
See the excellent articles at:
"Lightning protection of boats and marine facilities"