Hardboard base. Could easily be MDF/plasticard/cardboard/matte board etc.
Block of foam. Resin entrance but could just as easily be made from putty/clay/Miliput etc.
Small rocks (garden stones) around the base.
Liberal coating of sand and PVA mix.
Sprinkle on some grit before it dries.
Paint and add static grass.
Obviously you wouldn't need the resin figure on top, replace with more rocks.
Half an hour to make, over night to dry then paint with a big brush.
Cave entrance apeture is about 25mm high.