The abrasive paper attached to a plan surface is a good way to flatten soles of metal planes for significantly improving their performances. Usually I use spray glue that allows a valid bur reversible bonding so you can easily replace the abrasive when it is exhausted. The only drawback is the glue residue left on the glass plate that has to be cleaned with alcohol every time . An alternative way is to use double-sided adhesive tape which provides excellent grip and when removed leaves no residue on glass and results in faster replacement operations .
If you can't find a full sheet of sandpaper wide enough then you could join two narrow strips togheter.
. The tape thickness is minimal and does not affect the flatness degree useful to obtain a good levelled surface.