Fantastic work:cool Love it when someone is building with cardboard. Actually, to respond to some of the question regarding building models out of cardboard:
1: Good quality cardboard is still cheaper than plastic sheets (Syntra or others). If you buy a 2 mm thick cardboard sheet vs the same in plastic, your cost is going to be cut by half!
2: Paints easily nothing's different about the cardboard vs plastic. Cheap cardboard will need to be sealed prior to be painted, but that's it.
3: I still have 25 year old models made entirely out of cardboard: no shrinking, warping, nor deformations of any kind (and I'm working with Bigatures, not miniatures).
4: Easy to cut and also to "delaminate": you can reduce your thickness by removing layers from your main sheet. A trick you cannot achieve with plastic sheeting.
5: Quality material can also be sanded (min. 300 grit) to round angles and seams can be filled with regular compound and then sanded clean.
That's it, sorry if I forgot something...