What i did befor was you know you thoes blow-up things you put in your pool for winter time? Get thoes and blow them up i htink they are around 40-50 bucks and keep your volocity under 240 or els they will blow up when you shoot them i think my dad said so...so i dont know
Sonotubes will work if you can find em large enough and cheap enough. They are the casings used to pour concreate posts. You may want to coat them with some kinda of water resistant/proof paint so they don't rot over time (they are strong cardboard).
we built the field sunday i think its coing to be good we ended up with 23 bunkers the only bad thing is the field is not complety level.
i would have liked to use some of the things you guys suggested but we are poor around here thanks for the help