Well so far I've looted a Devilfish, a Wave Serpent, a Leman Russ with Basilisk turret and most recently a Monolith into a Stompa
I can upload pictures later if you like
If you have enough Orky gubbins anything can looks sufficiently cool, and I cannot advocate enough how useful the "Orky Fixins" upgrade pack from GW was to the Stompa...
As OD said, armour plates and battle damage are the way forward, as well as adding pretty much anything you can, my Wave Serpent has storm bolters and the 'lith/stompa is rocking a Rail Gun
Go nuts, have fun with it and remember.. less is definitely more, you need a balance cos a model which is too busy can ruin a perfectly good conversion