I received my California license plate the other day. Took just under 3 weeks to get it after I mailed in the form and cashier check.
I used the bicycles serial number that was stamped on the bottom bracket. I only sent $19 but it looks like it now costs $20, oops, but they gave it to me any way. :-)
On a bicycle you don't have many options at mounting locations. After checking out a few ideas and looking my bike over, I found two little holes on the rear above the axle that would be perfect for a custom made bracket.
I used a small piece of angle iron with two holes drilled in it, these would mount to the bike itself. And I welded on a small piece of flat plate with a slight curve to it. I drilled a couple holes in that and mounted the plate.
It works for now, and doesn't look to bad for a quickie job.
Constructive criticism is appreciated.