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^^^ those are the ones.
Can you bolt the whole assembly onto the bottom of your seat pan?
Watch out for clearance issues with the plate (and holder) and your rear wheel. On my first build I didn't factor enough clearance in and ended up with a busted taillight and chewed up plate. I had a hard time finding those P clamps Maritime's talking about but eventually found them at Princess Auto, in case you end up going that route -
- they sell them in a bunch of different sizes