Hi David,
I concur with Ashley here. Having made many many carry cases/boxes in my time, for myself and others there are a few things I would consider first.
You can't make the box until you have the model in your hands.
For a model of this size I would use 10mm plywood for the top/bottom/and ends and 3 or 4mm ply for the front and back. You have a rough idea of the sizes probably required, so why not get those bits in stock ready. You'll also need a handle, a couple of catches and hinges (I like piano hinge) All the hardware I get from Screwfix and the ply is available at any good timber merchants, preferably one who will do some cutting for you. I reinforce the stressed corners inside with some square pine strips, just belt and braces. and you'll need a good wood glue, PVA or white woodworkers glue (I use Aliphatic resin glue, the yellow one coz I like it!)
Other people like to use great chunks of chipboard or old oak flooring by the look of it, and while their boxes are masterpieces in their own right, they can hardly lift them!…The secret it to keep it strong but light after all it is to protect the model from dust and things, carry it to and from the car and allow your next model to be stored on top of it saving space without killing you or breaking your back.
Get this stuff ready and build a stand for it first when you get the kit, which will alter the measurements, make the stand removeable, and when you know for sure the exact sizes of the model, with or without a folding mast, you can build your box.
Cheers…..RON.