I've always liked the Shokk Attack Gun, from the first time I encountered them in the Epic Ork and Squat Warlords book. I had great fun blowing Titans apart with warp-crazed snotlings back in the day.
I love the current SAG model, but have yet to put down the money for one. So, instead I had the urge to try to convert one. At first I looked at a loota kit, but decided that the Big Mek should be at least nob sized, so started with a nob body.
Toggle switch taken from a loota frame, necron scarab, tau battlesuit head, some skaven warpstone thing ( I think from a doomwheel), bit of plate from a trukk or wagon to brace it on, IG lascannon powerpack.
The current plan is to paint the interior of the barrel like you're looking into the warp - pinks and purples with little white dots or something. Choppa for whacking gadgets or banging himself in the head to help him remember stuff. Stuff like: banging yourself in the head with a choppa hurts!
Mek tools and bosspole. Metal plate on his head for his bionic bonce/cybork body.