In a sudden burst of creativity, I went to the workshop today and built a shelf for the window. It's 52"x8" with raised sides; no real reason for the raised sides except I thought it would look nice, and maybe it will reduce the chance of pots getting knocked off. Installed, the window trim will form the 4th side. All the wood is scrap pine we had in the garage. The sides I ripped from a ratty-looking 2x4 that was much nicer on the inside.
Now that I know the shelf fits correctly, I just need to give it a quick sanding, stain it, and wait for the cast iron brackets to arrive. (I've discovered the joys of buying hardware on eBay!) And if it all turns out the way I hope, I'll make a similar shelf for my office window, and I can move some of the plants off the Majestic stove.
I mean, it's cute, but it is a *stove* after all.