I'm trying to think of something fairly simple that would work for that, and right now the best I can think of is to poke around Home Depot or Lowe's, and find some sort of plastic rod/tube/bar that's slightly flexible. Maybe some bit of fake wood molding, acrylic corner gaurd, something like a yardstick?
cut it in pieces a few inches wider then the tank, and flex them into an arch and wedge them against the trim, and then set the light on top of that?
Not even sure that will work, but it's all that I can think of right now that wouldn't really involve building/tinkering.
Aaaand, nevermind, I just read the bit about it being a rimless tank.
Well, this would be ugly, but should be pretty easy, and I think it would work. Not permanent, but you could get probably a dozen pairs of these from one piece of wood:
Just cut it into pieces about as wide as the legs for your light, fit the groove part over the glass, and rest the light's legs on the tongue part.