Sapper, think of it like websites, or shareware computer programs.
For the most part it is free, yet people can choose to pay. If noone pays it shuts down and eventually dies. Anyone who wants to see it keep going can pay, and to be fair it wouldn't cost too much even if it did come down to a core of 10 paying for it (and I reckon if you just asked for a whip round on here you'd get more than 10 people willing to chip in).
That does mean some people get a free ride, but in my book that's fine as long as they respect the place and don't leave it full of litter, burnt roots and mangled trees.
For the record, if I manage to get down there, I
will be contributing to the insurance (and if I had a bit more spare cash I'd contribute regardless, but unfortunately I can't afford that kind of generousity at the moment!)
If it really bothers you that some pay and others don't, you just need to "add value" somehow. Maybe set aside a few weekends in the year for "shareholders only" if that makes sense.
A mini-moot for those who have paid for the insurance and a guest or two each.
I think limiting it to just 10 (or more) people would be a shame, I can see people buying in just so they have the option, but time and commitments getting in the way and so using the site less frequently than another person might have done.
I think either an "optional donation" system or pay per use is the better idea (and I'd be half inclined to go for the former).
I think the "shareholders moot" would be a fun way to add the value too. Of course, anyone could come to that moot, they'd just need to pay a share to the kitty too - that way the following year's insurance can be covered just by a few extra meet-goers not wanting to miss one.