I have a small window that gets good light. problem is there is no ledge and a small old-fashioned radiator underneath.

I was thinking of building a radiator cover to put a few plants on top.

Is this a bad idea? I`m sure I saw this done with Orchids on saucers with water+pebbles...??

Welcome Corj! Yes, you can absolutely put plants on radiator covers, but you have to be careful. In front of ALL of my windows are radiators, and downstairs, they all have covers-old fashioned, steel(?) 2-piece covers. One advantage to having covers is that you can put a tray of water under the cover, creating more humidity. You'll also have to watch the humidity and moisture on any plants you have there, but it's not hard. I have my crotons, which I did have a small spider mite prob a while ago, but with regular misting, it's fine now, a cactus plant, mother of millions, lucky bamboo, and my pointsetta, which seems determined to make it into the book of world records-still in full flower on the radiator on the north. I think it may help, having things on radiators, as long as you're very careful about watering and misting-I think it creates an environment more similar to many plants' natural climate. Good luck!