easiest way to make ork vehicles is to trow a whole lot of bitz and plastic card into a box, add glue and shake until you get the desired result. but i don't think that it would look good as this though.
MORE GUBBINS FOR THE WAAAAAAAAGH!
just wires poking out here and there, useless glyphs and as many guns as you can stick to it, and if possible some sort of ammo feed, like take clips off shootas and glue them on top of the turrets.