You may be able to find a one-way relocation car rental that suits your needs. Potentially, if you find a camper, you could sleep in it as well.
Currently this site shows $5/day for one-way rentals, and some offer up to $100 fuel allowance. This would give a lot more freedom than other options, but the downside is you can't plan in advance so it may not end up being available when you need it. But if the reef is not a necessary trip then you could 'wing' it that way.
http://www.imoova.com/imoova/relocations