What about going to a lumber yard and have them cut a piece of wood to your exact dimensions? Then you can hammer on some small wood moulding, and paint it to the color you like?

I wonder if a granite yard would have small pieces like that? Call around and see.

I've used tidbit trays for houseplants, but they're not that deep or long.

My first stop for just about anything is always eBay.

"It measures 29" by 6" by 1" high. It is made of copper. It is made to put put flower pots in to catch run off water from the bottom of the pots. It is in very goo, pre-used condition."

Here is a link that might be useful: Long Polished Solid Copper Flower Pot Tray Liner Gorgeous

m3dogs' post and our discussion of repurposing got me thinking...how about getting a length of gutter and crimping the ends. They aren't very expensive for a three-foot length and copper is fairly malleable.

Here is a link that might be useful: copper gutter

You could also just get a sheet of copper and cover a piece of plywood cut to size... would look like these copper countertops

Here is a link that might be useful: copper sheeting

A plant tray would protect the radiator cover, and the floor below the radiator from any drips. Just google "plant tray." There are stock sizes and some places will let you customize them. There are plenty of copper ones available.

Here is a link that might be useful: one example of a plant tray

Thanks, all! Some good options here. Plant tray, of course, duh, that's what I should have been googling.

Just make sure that the plant tray you buy is constructed such that water doesn't leak through the seams. I once bought several copper trays from Smith & Hawken or a similar company, but soon found I couldn't leave the trays on our wood window sills - water leaked at the corner seams. I eventually gave away the trays.