Edge supports can be made from strips of paper
that are folded along their length. Thicker papers
can be used for larger items and thinner papers
for smaller,lighter ones. The strips should be
longer than the outer dimensions of the sheet and
the vertical strips should be cut at their folds
where the meet the upper and lower edges of the
sheet, so that the horizontal strips can pass
through them. This design allows the strips to
be secured at their ends (with linen tape), beyond the corners of the sheet ant that means that they can fit the sheet, snuggly, and the sheet can still expand, when relative humidity rises. This last point is extremely important. Corner and edge supports that are fixed to the back mat can cause the sheet to cockle, if they fit and if they are loose, the sheet can move out
of position. This is the basic strategy, but there
are many variations to meet the special needs of
different items.
Hugh