You could try here http://www.westhighlandretreat.co.uk/

I haven't been myself but it gets good reviews on the naked Scotland forums

Not in Scotland, but a few years ago we stayed at Fort William for a cycling holiday. I always get up early, and so I don't disturb the wife (we had a private secluded cabin)  I take the dog for a walk. As we were just a short distance from the Glen way, that's were I walked - as soon as I was on the path - off with the clothes and a very pleasant naked 4 mile walk - great way to start the day - never saw anyone at that time in the morning.

Just back from a weekend at West Highland Retreat and I can't rate it highly enough. It's a great B & B but also has space for tents and campers and the use of indoor toilets etc., plus breakfast and evening meals if you require. It's midway between Fort William and Mallaig on the A830. They have a web site and advertise in HE. They have recently added a hot-tub and a petanque court. John and Jan who own and run the place practice what they preach and are ardent naturists. It has fantastic scenery as a back-drop and John can show you where to naked ramble.