Tools & materials:
Coping saw
Wood rasp
Chisel
Dust mask
MDF board or thin plywood
Acoustic ceiling tiles
Small pieces of wood
Glue
Sand
Paint
I used acoustic ceiling tiles because there were a bunch in the garage when I moved into my house. Foam or similar would work too.
1. Glue a block of the tile to the MDF or other basing material and allow to dry. Make sure you take into account the size of your miniatures as well as some amount of extra for the walls. I drew a trapezoid big enough to fit 1/72 medium field guns and added about 2cm all around for the sides.
2. Use the coping saw to cut through both the MDF and tile and shape into whatever shape your final pit will be.
3. Use the wood rasp to round over and shape the edges of the tile. Wear a dust mask as this step is extremely dusty.
http://i.imgur.com/WC9EL.jpg 4. Use a chisel to carve out the pit. The tile is very soft so if you chisel is even halfway sharp this should be easy. You can even up the edges of the base if you have a freshly sharpened chisel.
http://i.imgur.com/DZ1SZ.jpg 5. Use one of your miniatures to judge how deep to make it. Round over the edges and clean everything up. Coat the tile in glue when you are done shaping it to increase durability. The tile is very porous and so dries quickly.
http://i.imgur.com/ib6xX.jpg 6. Cut small pieces of wood to cover the bottom and glue into place. I cut them somewhat roughly and at different lengths. Put sand on the wet glue between boards.
http://i.imgur.com/aE0s3.jpg 7. Do the same for the sides. I glued horizontal cribbing in place, then used bits of wood and toothpicks for braces. Add more sand.
http://i.imgur.com/ENTi6.jpg 8. Spray paint the gun pit black and dry brush with brown, light brown then cream colored paint. You could add static grass, flock or snow to the base depending on where you’ll be using it. I left mine dirt colored for now.
http://i.imgur.com/2S8le.jpg http://i.imgur.com/UKYRc.jpg