For LEDs, especially those without gel frames I like DC Fix. It's a self adhesive film meant for windows, but it works well for LED fixtures since you can just cut it to size and stick it on.

Another thing I haven't seen mentioned is putting a surround around them. Could be a 3 sided plywood box, or a piece of sonotube cut in half. You'll have to play with the height and sizing a bit to get something that doesn't keep much light off the walls and is tall enough that guests don't actually see the fixture unless they're standing close to it. Once you've got that figured out you can paint it, cover it with fabric, etc.