Quickie update #2:
I applied more plastic card in various thicknesses from 0.25mm to 1.5mm. Then roughly applied bondo over all of the joins.

A note for those thinking about giving bondo a try for the first time,
it dries in less than two minutes. This means once you mix the two parts of standard bondo you have about a minute to slap it on the model. For larger complex pieces like this one I had to mix a batch, apply it and repeat.
An easier solution is the
bondo: glazing & spot putty. It is premixed in a tube and you just squeeze out the desired amount. This way is more expensive for less product but you will end up wasting less and there is nothing really to clean up.