At multiple events, I have seen a miata with it's class and number printed on basic printer paper, inside a clear sleeve, and taped onto the car. I really like this idea and I want to do it my self. Unfortunately I have not had any luck getting a word processor to come up with a suitable font for narrow, tall numbers.
I would suggest that you keep on searching since there are at least 30 fonts on Microsoft's Word Processor since the 2003 version and there's at least 1 font that is good enough for narrow numbers. It's certainly cheaper than buying numbers from a vendor.
Here is a link to the complete solution.
http://www.ccrscca.com/SoloNums.pdf
Duke
UltimaDork
7/14/14 9:15 p.m.
Or Photoshop Elements, which comes with some cameras.
wbjones
UltimaDork
7/15/14 6:02 a.m.
JohnRW1621 wrote:
Here is a link to the complete solution.
http://www.ccrscca.com/SoloNums.pdf
you're all the way up in Ohio … how the heck did you know about CCR and their printable a-x numbers ? 
Easiest answer: GRM
Longer answer: I had a Subaru site bookmarked on a previous computer that had same/similar numbers. It is something I came across on GRM (I have never owned a Subaru.). Having since switched computers, I no longer had the bookmark so the magic of Google brought me to this link given here.
Full confession: I have no idea what CCR stands for or even what state it is in. They due however have some nice numbers.
wbjones
UltimaDork
7/15/14 6:59 a.m.
Central Carolina Region SCCA
typically …the Charlotte area (though the region encompasses most of NC and SC)
Print your own numbers provided by HSCC. Look towards the bottom for a PDF of legal numbers and letters. I think we borrowed them from CCR. FYI, they will fit on the magnetic vent covers you can find at your favorite hardware store (~$5/3 pack).
Instead of a clear sleeve, you can get magnet backed printable material - I ran numbers like that for a while.