If you stick (no pun intended

) with a 3-digit number, you will have a lot more options as to magnetic decals (e.g., rectangle, rounded rectangle, oval [see avatar], etc.).
I got my magnetics from Final Touch Graphics - 2 sets of numbers (10" high digits, oval, silver lettering on black background), 2 sets of class designators (rectangles, 6" high - separate in case I want to change cars/classes), and even a nameplate for the driver's door (a must have for a serious bench racer

) - all for about $110 including taxes.
If you want to go fancier, check out the different magnetic styles at the next autocross - some have highly styled decals (waves, slanted, etc.). You can also see most of the styles by looking at the photo postings from the last autocrosses (see "photos are posted" threads in this forum).
Getting magnetics made usually takes 1-2 weeks, so plan ahead.