I think a Mercator should come close to the dimensions you need? It distorts pretty badly towards the poles, but unless you have anything significant going on there it shouldn't matter too much.
Alternatively, you could use a 2x1 (or other non-30x24) type map, but fill the remaining space with decorative or informative graphics. A lot of antique maps do this, see for instance here and here. It might help add some flavour, and solve the problem of the dimensions. Two birds, one stone?