Hello!
philo - I'd jokingly say I'm sure you are as desperate to make that conversion as the German troops were in the eastern front in 1943! So I'm sure you can do it. I think what you are looking for is the so-called Moebelwagen:
https://en.wikipedia.org/wiki/M%C3%B6belwagen
In this baby you only add five flat armor panels to the Panzer IV hull.
The prototype was armed with a Flakvierling, the regular production vehicles with 37mm Flak, so you'll probably have to take some liberty here...
Hope it helps, have a nice day
Paweł