I think I must respectfully disagree. With the "same part flanges" you have 360 degree contact multiplied by how many flanges you have holding the bungs in place, and because they are oversize and flex they are likely to grip strongly with quite a lot of force, and continue to do so if removed and refitted. Also because they flex they take up any variations in size, paint thickness, etc, and any one of them is probably going to make a waterproof seal, but you also have redundancy with multiple seals.
With Mark's possible solution the interference fit of the ribs is what will be holding the bung in, so variations in size will be more critical, and if these ribs flex sideways even slightly, the outer surfaces will become angled and you will then have less material contacting and gripping the inside of the frame tube, unlike the ring type where the more they flex the more material is in contact to provide grip. They are also less likely to grip as well if removed and replaced. The ribs will also provide no sealing, so you have the complexity of an additional part, the O ring, but unlike the flange type, with this the sealing depends entirely on the bung remaining in hard contact with the O ring and not working its way out even slightly.