If you use a mount cutter it should not be or look crooked. Before mounting, use your straightedge to the bottom of the mount, check it for square; and when you stick mount to page it should not be crooked.
My mount cutter is not as long as this piece, so it may take me a few tries to get this right. Or, perhaps I could (for this piece) shift to a clear mount.
To rogdcam's question, the piece I have in mind is an eight-stamp vertical strip (a booklet pane, I guess). Like this:
So it's definitely longer than 215 mm.
Thanks for the advice, everyone.