I too feel a flat field would benefit from an extensive trench system, it would be almost unique ?, BUT safety must be paramount, for starters an experienced digger operator is necessary , an amateur on a borrowed bit of kit would likely cock it up.
plus even fairly shallow trenche would be subject to wall collapses over time, especially during use, so some kinda shoring up system would be necessary , prob timber, & there's the water table to consider, so close boarded pallets as running boards may be needed.
i love the idea, done properly it could quickly develop in to a viable airsoft business, which in turn could bring more funds for site development, but you can't afford to bodge it, even if its just a few mates, somebody buried in a trench collapse would spoil the day lol.
reccomend you start stockpiling pallets from any source you can.