Method:

Cut a 2,5 inches by 2,5 inches square of thick cardboard. Draw a line 3/4 inch from one side (on both sides). This is where the ladder will be.

Make some thin cardboard circles with the holepuncher. Depending on the sort of rivets, you are using, the size of the circles can be large or small. Make some geometrical figures and strips from thin cardboard. Cut them out with a sharp knife. Decorate the larger area on both sides. Layer-on-layer creates a good effect. Remember rivets.

Create a latter in the narrow area - make the steps out of either matchsticks or staples. Use superglue to attach the steps, unless your patience is greater than mine. GW bulkheads have 7 steps. Glue on a H-beam on the top and an H-beam or an I-beam on each side. The H-beam on top has to be long enough to cross the I-beams on the side. This is where walkways and cardboard buildings will be attached, so the H-beam is important. Glue an I-beam on the bottom of the cardboard bulkhead. If you want to save the I-beams, then use a strip of thick cardboard, about 3/8 inch wide.

Paint to fit your usual Necromunda colorscheme.

As a 6-pack of bulkheads cost 6$ by GW mailorder, there is money to save, especially if you somehow find a free source of H- or I-beams. For I-beams, I can recommend looking at the "spoons" handed out with a cup of coffee at MacDonalds or similar places. The handle is about the right length. And, the "spoon" can be used as decoration for the bulkhead (pressure window).