[Inkscape-user] How to make a mask?

I've been trying to work out how to produce a mask, to be used for compositing an image with ImageMagick by a CMS.

What I need is a white background and a rounded border (with a stroke of one or two pixels), then with the middle punched out so an image on the layer below shows through the hole. I've tried making three layers, one solid white, one with a rounded rectangle with rounded corners with a transparent fill, and a copy of the previous with no stroke but with a solid fill, and tried various combinations of 'mask' and 'clip' but I can't seem to find it.

Can someone help me before I resort to chucking it into Photoshop and cheating?

Paul Bolger