Bill, the number and size of the wording on the brass plaque will determine the size required so best start with that and then you may be able to fit that adjacent to the plane profile without adding too much more area.

Emgee

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