here's what I did with the bottom of a toy car ( which I almost threw away), card , 2 plastic screw type lamp holders and other scrap plastic.All doors have tube + wire hinges and the rear one has an extendable piece, Engines are made of 2 screw type plastic lamp holders + card.I used 3mm magnets first but these were not strong enough to take the overhang so I used 7mm ones.
The under carriage struts are made from GW titan weapon arms and dismantled disposable blade heads glued upside down.
Here are the photos one after the other so you can see how simple it is to build something that you can say " I did this "
Still have to glue everything up and I can't wait to paint this one.
I put 15mm minis next to it but it can easily be used for 20 or 25mm.
Enjoy and as always may you be inspired.