[Inkscape-user] Make document square; keep contents centered

I feel like I must be missing something *really* obvious, but I can't figure out how to do this. I have a rectangular document that I need to make square, while keeping the contents centered. I.e., I need to add equal amounts to the left and right side of the document.

How can I do this? I've tried simply entering the desired margin sizes in the Document Properties dialog, but it has no effect. Changing the width of the document to match the height extends the document to the right, but it leaves the contents in their current position, which is now shifted to the left.

Thanks!