...the members all pay a monthly mandatory donation to the club of 20% of what their miners made that month or flat membership dues...
Good luck with that. Miners already get uppity over fees of 1-3%. Adding an extra 0 is sure to go over just fine with them.
I would agree on 20 I highly doubt many will get interested. Fact is like eleuthria said miners are known to move pools over much less fee.
Now if you go co-op where everyone paid based on what they have there not percentage makes more sense. My ISP being a small town is a CO-OP. Our pay really is not high considering price of 1 gigabit internet. And at end of their financial year I believe profits are split to it's "owners" which is everyone who uses it. But considering you paid the money in the first place you don't really make anyting.