You might find a round knob might have clearance where an oval one didn't. Then, while not as handy, they do sell the valves with a removeable key rather than a knob. You could probably get a couple of positions where the key could be engaged to avoid it hitting the floor. But, if this is either in the wall or coming out of the floor, the better choice would be to replace the valve. If it's coming out of the floor, BrassCraft sells a chrome plated 5" valve that slides over the copper stub, putting the valve up in the air. If it's coming out of the wall, they sell right-angle valves that would raise it away from the floor. Dahl makes a nice one, and probably others. It depends on how the existing one is attached what your options are. If it is threaded on, you may not have enough room to unthread it. If it is either soldered or a compression, you should be able to get the old one off and a new one installed fairly easily.