Do you have any idea if the 10 sqm that was mentioned before is still the limit? or is there another number floating around out there? If so, it may be neccessary to go with a building permit to allow for more usable, larger structures (like the Drug Lab from Flag Raiders for example). Other than that the "L" shaped idea Brian shot out was a good one, maximum possibilities for play and enjoyment on a strictly limited area. I would be more than willing to CAD up some drawings if the time comes where they're needed.
Also, the idea of a membership only field is a good one, it essentially eliminates the potential to sue. Have a waiver to sign upon acquiring that says something along the lines of "By signing this waiver you forteit the legal right to sue the host.". It may not be the flashiest of ideas, but a solid one. I do however see the need for walk-ons and "Joe Shmoes" to come to the feild, providing that extra revenue you may need to keep the field operational. If they go through a pre-screening, similar to a sports team tryout where we can gauge their attitude and seriousness it may help to weed out the bad seeds.