SS PANZER:
For your idea. If you want to reproduce the gun pit from Band of Brothers the previously suggested insulation foam is the way to go. It's cheap and easy to sculpt into the shape you want. For reinforcement as shown in the film you can utilize a few different methods.
If you watch the assault on the guns you will see what I am describing here
1. Use toothpicks or small twigs found locally for the upright pieces. The weave part can be done with old cassette tape sliced thinly and woven in and out of your uprights.
2. Use toothpicks or small gauge wire for the uprights and weave small gauge wire in and out of the uprights.
When you are finished you will have the "support" required to retain your mound. Glue in place and top with sandbags (do not spoil your hard work and use commercial kit sandbags) Make your sandbags from tissue and real dirt or use Milliput or similar epoxy putty.
I hope this helps.
Cheers;
Gregory