You are too generous ()I give you 10/10 for posting these pictures.
Andy
Could you not have put the side panel onto the frame before fixing the frame itself in place?
In future:-
Boxing is not structural. Make it up on the bench with glue and hidden internal fixings if required. Hold it in place with a couple of small dabs of grab adhesive. It won't move but is easy to remove if you need to. Sometimes you don't even have to secure - some of my boiler boxing just pushes on to a couple of dowels.
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