Ork Trukk with Krooz Missulz (aka an Orkish version of the Imperial Deathstrike Missile Launcher, but why not use the Krooz Missulz rules for the Mega-Gargent.
This is a very simple conversion that I did a fair few years ago.
The model is built using a Flakwagon base, a firing platform from a Pulsa Rokitt and the Krooz Missulz? Well it’s a 40k scale Fighta-Bommer Missile.