"The actual opening size of the GL300 is 300 x 300 and not that as described in the main heading of 420 x 410. Having cut a hole 400 x 400 before receipt of the panel I was far from happy to find it was useless for the job and had to be returned for credit. Where the 420 x 410 dimensions came from I am at a loss to understand since even the outside diagonal is less than this !!!!!. I suppose the manufacturers model number of "GL300" should have been the best indicator of the size. This is the largest panel that Screwfix do so I ended up having to make one from scratch."