As many have already commented, teak is in many ways a good option. It is not particularly cheap, but you might be able to obtain offcuts from a small business that does a lot of work in teak. However, it is worth bearing in mind that while teak is used extensively on board yachts, this tends to be in the open air. You may find that in what may be the probably somewhat humid and not particularly well ventilated environs of a "heads" cubicle any timber will remain damp for longer than is desirable. I would suggest at least investigating the possibility of making something up using plastic, as used in the building industry. Many years ago, I replaced a rotting wooden base in a gas bottle locker on a yacht and decided that plastic was the best option; it was still in excellent condition when last seen just over 4 years ago. The plastics can be obtained in various forms (square sections, slats, etcetera) and probably colours - white is common for both buildings and yachts, so I didn't investigate further.
Tom
