I am not very clever with this sort of thing but a cereal packet cardboard mockup of what you think might fill the bill, always helps decisions. If I were doing this, I would start by making the plate first as suggested above, then arrange it in a pleasing position, then set about making the cardboard test piece allowing at least an inch, perhaps one and a half inches front and rear of fuselage, hull or whatever the model is and see what that looks like. Trim some off or sellotape a bit more card on til I was happy with it. Similarly with the other 2 sides, adjusting sizes until I was happy with the whole arrangement. Next, find piece of wood to replace cardboard. If its light coloured, a damn good sanding and a couple of coats of good varnish on the wood and the metal plate to prevent tarnishing. Maybe use 2 or 3 thinner pieces of wood of slightly different footprint glued together creating a stepped effect would enhance the appearance of the plinth. Have you thought about a glass or perspex case for dust exclusion? Good luck with project and give us a piccy or 2 of results.
John