When you're facing Daleks, it's handy to have some extra firepower, so I built a cybergun. I started with a central piece cut out from a profile drawing of the gun (see here for example). Then I added foam to each side to give it width. Then I glued more beaverboard pieces on, aligning the edges I wanted to meet up (you can practice with cardboard first until you get the dimensions you like). Then all the seams were filled with thermal glue and wood filler and the whole thing was painted over. To make the pump action, I used a Dremel to cut out a few slots in the base, and added some plastic pieces that were about the right shape for the slider.