If this is the look you want, it can be achieved. But you will need to buy the right parts and finding them alone on e-bay might be costly.
First you need to get the right KO rocket launchers. If you bought the set with the gun you probably got the wrong type. Unfortunately the guys who KOed the rocket launchers and gun set are not the same guys that KOed the whole City Commander trailer set so there is a mold difference in the rocket launchers. The launchers that come with the gun are an exact duplicate of the authentic Fansproject launchers. The ones that came with the KO City Commander set have a notch in the back. The notch really helps.
You will need to get hold of two rafts from ROTF Whirl (TRU exclusive w/Bludgon) or ROTF Evac. The rafts snap apart into three pieces. You only need the bottom pieces of the rafts with the posts.
Turn the pieces sideways and wedge them into the notch on the back of the rocket launchers. Then just pop them right in Ultra Mammoth’s shoulders. If you want to sacrifice the rafts and the KO launchers, you can glue them together into a position that looks even better and more compact than how I wedged them. And if you use the glue you don’t need to have the launchers with the notch. You can just glue the raft right on top of the rocket launcher post centered with enough clearance to plug in the raft posts to the shoulders.
I think it looks cool, but I don’t have a set of white launchers with the notch plus I don’t want to destroy the rafts. If you decide to try this, post pictures so I can see how it looks…Peace.