on of the clever bits of 'field engineering' I picked up in the service was this: take a three foot (or more) length of 4"PVC pipe. Bury it vertically as deep as you can, and then put eight inches to a foot of coarse gravel in the bottom of the hole. Before you backfill, trim the pipe so only about three inches sticks above grade. Now, you have a pipe into which a 2x4 will slip and it should be snug, but not too snug as it requires a bunch of cutting. When you're done for the day, put a cap over the 4"PVC pipe (you can even attach the caps with a string to keep them from "wandering away." To keep stray shot from chewing up the part of the PVC that's above grade, use a sandbag (you remember sandbags, and the endless need to fill them, right?) ... put the sandbag right in front of the PVC, and all that will get shot up will be the 2x4.
Good luck with your project!