I got a load of 2mm plates cut to the ATL blank 4″x6″ size.

Sold most on ebay which left me with the remaining ones “free”. 

These were then attached with countersunk rivnuts to the sills

I added some washers behind the rivnuts as the countersinking left the panels a little bit thin.

I’ve not bothered to cut “through” although the centres are drilled for if I want to.

Bonus points for spotting a school boy error here though. 

Finally a picture of the sill with them (and fake ribs) in place