you should be able to pick up a multi-size pack from any hardware shop, mitre 10, bunnings etc. just ask for metal or tube spacers, if you can find rubber ones there I'd get those & cut them to size until you find a fitting you're happy with
take your plate screw with you to find the right size so they don't stick out too much, you won't notice the gap after awhile anyway
no way my rear plates would fit without them