I intend to have a fairly traditional multimeter external design, but with a few changes:

For a start, I don't have access to custom LCD screens like commercial ones do, so I can't do that. I do still want to have multiple pieces of information displayed at once, so that eliminates a 7-segment display. I will probably go for an alphanumeric display, although a graphic display may actually be better (that way I can have varying text sizes and such) – I'll look into it further when I get to that stage. The alphanumeric display gives a minimum width, and unfortunately it is wider than I'd like (especially when I allow as much room around it as I did – that concept render is 120 mm high and 100 mm wide).

I'll stick with a mechanical rotary switch for the main function switching. I'm not sure whether it'll be something I buy, or whether it'll be some tracks on the PCB as in most if not all commercial multimeters.

For any buttons, they'll be above the rotary switch, on either side of it. I'm currently thinking of capacitive touch sensing, with a dip or ridge in the case to show their location.

For my prototype I'll make the case out of wood. What I'd really like is aluminium with a suitable insulating layer inside, but I don't think I can manage that (I could mill it out of a solid block, but that would be extremely wasteful and also expensive). But whatever it is, in addition to looking OK it needs to be reasonably strong (both for if it gets dropped, and if a fuse blows with a very high current) and have the outside well insulated from the circuit.