That's easiest - mine are just black magnetic sheet cut with an Xacto. Took about 20mins and cost maybe 10-15$ A 24" x 12" sheet is probably enough for 2-4 numbers (1-2 per side) and 4 letters (2 each side) if you're conservative with it - 24"x24" if you want larger numbers than minimun size.
Even less thought involved; buy a stencil and some krylon and paint the numbers onto magnetic squares.
Just avoid the thin printer ready magnetic sheets - they fall off at speed.
Also - if the magnets ever warp (my eternal problem), toss them on the hood after a long drive, the heat will get them to lie flat again.