toonie, i got boards shorter than the envelopes, but i don't recall whether i ordered a specific size that was already offered or specified a special size when i was on the phone with them. these are the ones that are the same size as the envelopes i used, and you can just ask them to cut them an inch short ~ which i recommend, because the silk is a lot of bulk in these so cutting down on extra contents is a good thing.
by the way, i also suggest ordering a roll of clear tape ~ with the bulk of the textile and many (MANY) openings and closings of these envelopes, i found the glue holding the envelope to the window failing along the top seam of quite a few. so i've just reinforced them all with clear archival tape and no more problem ~ just one strip of tape along the seam and wrapped around to the back side to make it look neat.
Stiffener Boards For Side Opening Envelopes
Cat # Description Size 1 or more 3 or more Qty
SEB91S 1 Pkg. of 10 9 1/2 x 11 3/4 $4.70 $4.25