Take a bunch of cheap plastic containers and cut and glue the piecies together to make a gun similar to make a cardboard gun , but in plastic.
The easiest solution is to find cheap toy guns at a thrift store/Goodwill/garage sale, and they mod them with other bits and pieces laying around the house--cannibalize broken computer bits, milk jug lids, out-of-ink pens, etc. Glue/screw them onto the base toy however strikes your fancy, and paint the thing. Black spray paint is the prop builder's friend. Then touch it up with silver (for worn areas).
Go build a non-firing knex replica. Screw that, go build a knex replica of a real gun that actually shoots.
Get something you can make a mold with, like latex, silicone or alginate mold compound. Make a model of a gun, or get a toy gun. Make a mold of the gun. Fill the mold with resin and allow to cure. Remove resin gun from the mold, finish and paint as desired. Repeat. Or build a vacuform machine, cut a model gun in half, make a plastic shell of each half of the model gun,glue the two halves together, finish and paint as desired. Repeat.