With the Panzerwerfer, I stopped rebulding a second version of the chassis and used the original Dragon Maultier chassis instead. Then I added the hull parts from the Italeri kit parts and some scratch made brass sheets. Finally, I added some scratch built interior. As this will be barely visible at the end, it should be fine the way it is done. The rockets for the Werfer were copied in resin:
There are at least two interior sets commercially available (Royal Model and Verlinden), but both miss parts, e. g. the structure in the upper half of the hull on which the rocket launcher rests. So I built everything on my own, using the few parts from the Italeri kit.
In the mean time I finished the interior (that will be barely visible on the finished vehicle):