Attaching trailer plate to bike rack

Following on from discussions some time ago about having to use a trailer plate when carrying bikes on the Nuevo because of lights being obscured.  Because of one thing and another we haven’t taken the bikes with us since, however we are ready to go very soon.

We want to use a cover for the bikes, so we will obviously need to attach the trailer plate outside of that.  It sounds a silly question, but what have you found is the best way to attach it?  All we can think of at the moment is stretching some luggage elastics round the sides.

Any suggestions welcomed.

Edit, sorry posted in wrong forum.  Please ignore here, & I’ll repost in coachbuilt.