I was last looking at taking the barrels from the 3rd edition starter and painting them like toxic waste containers and slapping a lit dynamite fuse on the bottom. They seem difficult to sit that I may need to scrap the idea unless I make it a two-ork operation. Nevertheless, i'm still in the concept phase unfortunately.
I do want to +1 the "scratch building" comment, as well as suggest checking out all the youtube builders who made new units from an extra trukk sprue. My fave is the wartrakk video