There is a Lucas "reproduction" fender mount tail light/license holder on ebay occasionally and have seen them under the counter at Vicious Cycle and Western Cycle salvage...about $25-30. I had one on a 750 w/550 rear fender, didn't have to drill new holes, just shortened up the fender by rotating it forward. Low profile and stylish.
Since you're in Portland, check out Portland Oregon Old Motorcycle Association (POOMA) on Yahoo groups. MrBlasty turned me onto it and it's a great bunch of local folk to ride with.
Welcome to the neighborhood--Mike