Grid Board Marking

[Lord Kermit of Birkenhead]

But wouldn't the sheep dogs round em all up....

[mmcv]

No idea if this would work but what about pin pricks in the base sheet? I.e. use a thumbtack to mark the corner edges by making a small hole. May be visible enough without being intrusive? Or lay out a 3d grid under the mat so there's only small mounds at the corners when the mat is laid over? These would probably only work with a light sheet, wouldn't recommend with a mousemat style!