Rob wrote:

.... Any ideas on how to replace the zinc without
shelling out for a short haul?

Sounds like you're comfortable in the water so I'd try getting the zink
ready to slip on, then hypervenelate, dive, and get it on with at least
one screw. Finish in subsequent dives. Alterantely, look up somebody
with scuba (or equivalent) gear to do it for you. Good Luck.