I cut a piece of sintra and drilled a hole in it big enough to stick my finger in for easy removal.
Then I put a few very small strips of Velcro around the lip of the opening.
Seems to work fine so far.
On my MOW pack i cut a piece of 5mm plastic card slightly over size then sanded to a precise fit, bottom edge in first then tight to the top so no fixing required, just requires a penny to pop it out...
I have a peice of sheet metal cut to size, with magnets at each corner holding it in.
I've just done my Geonosis Jango JP Pannel.
0.8mm mild steel, held in place by 1mm (STRONG) earth magnets, then covered in Rub N Buff.