More likely eels...
More seriously though Jon, if you're envisaging significant numbers the needs include:
a) 240VAC power;
b) toilet facilities of some sort;
c) potable water (tank) and somewhere to eat / drink (even if it is BYO);
d) closely followed by garbage disposal;
e) somewhere for people to warm up - a small cabin with a heater and something to sit on is ideal - away from the scopes where a little light won't bother the observers;
f) somewhere for those wanting a sleep to flop before driving home;
g) insurance.
In effect it amounts to a camping ground with say some unpowered sites for tents, powered sites for caravans, a communal facilities block with a kitchen, a heated lounge room, toilets and a couple of showers, plus a separate area for setting up scopes away from the people and lights/noise.
If you can manage that lot you should charge a few $ per head per night.