This is the way i fitted mine, makes it easy to remove if you,ve bonded the side panels on
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From inside
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Very neat work and a top tip

Good idea

The way i did it was to loosely fit the side panel, fit the rear wheel arches then remove the arches and side panels, the plates are made of 25mm x 3 or 4 mm bar about 50mm long drill and tap one hole the size of the bolt you're using to fix arch drill another hole in the flat bar , bolt the flat bar to the inside of the side panel then drill through the fibreglass and fix plate with pop rivet I used 4.6mm large flange pop rivets, doing it this way has 2 advantages 1) you can remove the wheel arch if needed 2) it spreads the load on the fibreglass
Tony

Good job, looks really neat.