Let's get down to brass tacks. First, grab yourself a suitably shaped container. The best thing I have yet found for bunkers for 40K is Swedish Glace's vegan ice-cream tubs (see Fig. 1), so that is what I am using here. With minimal alterations, these instructions will probably work with whatever plastic containers you have to hand.
1) Cut away the bottom of the container, and carve vision slits into the sides of the container. This sort of plastic is flimsy, so be careful not to gouge yourself. See Fig. 2.
2) Test fit the lid of the container on the bottom of the container. See Fig. 3. The tub I'm using has a loose fit, which is exactly what you want.
3) Get some 5mm thick foamcard. See Fig. 4. Cut it to fit the interior (or exterior, if you prefer) sides of the tub. In this instance I cut rectangles of 65 by 50mm, one for each of the sides I wanted to bulk out. See Fig. 5.
4) Glue alternating panels in place with a hot glue gun. See Fig. 6.
5) Trim the remaining panels to fit into the gaps, and then glue them in place with the hot glue gun. See Fig. 7.
6) Extend the vision slits you carved in the tub through the foamcard. See Fig. 8.
7) Knock up a door from some spare bits of plasticard. See Fig. 9. Glue the door in place, and use some foamcard offcuts to make a set of steps up to the door, if necessary.
8) Glue sand over the structure, taking particular care to avoid the door and not to block the vision slits. This sort of plastic does not take kindly to glue, so you will probably need to take a couple of passes at this. See Fig. 10.
9) Paint and you're ready to go.
As you can see from Figs 11 & 12, while a standing infantryman has "line of sight" out of the vision slits, the same cannot be said for the barrels of heavy weapons. My next article in this series of terrain tutorials will cover a few variations on this general build which can offer something potentially more appealing to the eye or gaming table.