I cut a crater shape out of insulation foam using a hot wire cutter & hot glued it to the base.
Using a butter knife, I placed the edge against the foam & dragged it downwards against the 'grain' of the foam to roughen the surface.
After a couple layers of paint it's starting to look a little like a pit. (The recently impaled dudes doesn't hurt either :-p)
But that's not nearly enough dead guys, so I got a lot of bits together & painted to fill the pit. I covered most of the bottom of the pit in PVA and then put the body parts into that. This gave me time to rearrange them into the proper places before the glued dried. Then I covered it in borwn pigment powder to help hide the glue & help the bodies look dirty & gross.
To add some more nastiness, I soaked some pink thread in a red & brown wash and layed them around to look like entrails. Also added some ballast as some filler.
For the outer crater base, I covered it in coffee & tea grounds added planting bark rocks like on the hut & watchtower.
For the final blood & guts to fill in the gaps I mixed together some red, black, brown & purple washes with some 3 dimensional adhesive called Diamond Glaze. It dries clear like PVA but doesn't shrink as much. Pretty sure I got it from a Hobby Lobby.
After that & some foliage on the ground the pit is done.