Part# KB-MSP-20-40-MCYL-CVR-KIT ; Kustom Bitz underfloor brake master cylinder floor cover kit.  Comes with a 2mm thick cover plate with nicely rounded corners and slotted holes for ease of fitting, a 4mm thick frame plate to match with taped holes, and 4 x M6 bolts.

Suits double chamber underfloor master cylinder. Hole opening is 175mm x 95mm.

We use this kit in our workshop on both flat floors and floors with complex bumps and bends. The idea behind it is to give you a good framed opening that looks good from the top and underneath with proper taped holes and bolts. On flat floors you can just sit the plate flat up on the bottom of the floor so that it sandwiches the floor skin between the cover and the frame, then weld the frame plate to the floor. This gives you some leeway on not having to be so neat with your hole cut, because the frame and cover hide it. On bent floors with ridges and swages, as you can see in the example photos, you can shape it and cut it in, weld it up, and then dress sand it up, to give yourself a good flat surface for a good seal.

This is a much better soultion than your typical hacked out flat bit tin with some self drilling screws.