Just finished building this field at our place today. It's only 200x150, but our primary client base is private groups of ~10 players so this will be a good size for that. Should be playing it next weekend. I used off-the-shelf corrugated roofing tin for the wall, PT 4x4, 2x4, and fence pickets from Lowes. We already had the plastic pallets on site. Everything is new materials and built well with a goal of low maintenance.
Theme fields are popular these days since most kids play FPS games. Just come up with a plan/idea, draw it out, walk the field to see what natural elements you can use and build it right. Once finished play it for a few weeks/months and tweak it if necessary.
Hopes this helps. Good luck.