The reason why I suggested the DIY wood with mouldings route is because it is so simple when you've got the tools and routine - and the parts costs pennies!
I've built myself a pretty descent wood workshop throughout the years, mainly to cover the bits I need for the never ending house restoration project but also for running repairs.
I've got custom size windows on my house and this is an expensive problem, that's until I learned to make my own windows that is. It's a fairly quick job with a proper table saw and a bench router (with the correct bits).
Ok, we're talking a few hours per window frame and the first ones I made were debatable whether they were cost effective, but I made them myself and that's also worth something!
Today I can make them a lot quicker and I recover the glass from the previous windows so I don't have to use modern glass either.
If you haven't got a bench router, or a hand router, you can buy cheap mouldings and glue to a wider piece of wood to create the right look.
Making the plinths in hardwood is the best but if you're going to paint them, or heavily stain them, you're fine with ordinary pine - which costs nothing!
Also making your own display plinths/stands should add to the "I made this" feeling to the whole project!
My Schwerer Gustav diorama is going to be built as a furniture due to its size and paying someone to build the custom display cabinet would cost a fortune whereas doing it yourself will mainly just cost time, and some material costs.