motorcycle license plate mount

[69harleysport]

I was wondering if anyone knew of ANYWHERE that could make me a license plate mount for my motorcycle. It's a custom 1969 Harley Davidson sportster.
The rear end has solid struts, not springs/shocks, so it's kind of like a ridgid frame. It needs to be able to withstand tremendous vibrations due to lack of suspension. Any information would be awesome. Thanks a lot!!!!! Adam