Far and away my best fit is CB3000 tube with .12 gap, 2.0" ID CB2000 style ring. The CB3000 cage ID is actually too small but it can be stuffed in and I can always use a urinal. The problem is hygiene. DOES ANYONE KNOW OF AN "OFF THE SHELF MODEL" WITH A More open, better hygiene and LARGER TUBE AND SMALL GAP? The smallest gap I find is at least .25".