I have a good peice of furniture for this - it's a low, hourglass shaped plastic stool for kids - the sides are concave, it's about a foot in diameter, and a little less than Eighteen inches tall - when you put it on it's side, it forms a saddle of sorts, and a dildo could be placed anywhere along the curve, which at it's low point is about Eight inches above the floor.

I tried to google up a picture of it, but it appears to be elusive - I found it last Christmas at a discount store, the hourglass portion is two pieces attached at the middle, with a plastic lid. The plastic is slightly translucent, it came in several colors, and appears to be reasonably sturdy - I have no idea where you'd manage to find one, if I figure out where to get them, I'll update.