It is a good thing to think about because differences in agitation do have an effect, but the question is - how much effect would this difference make?
Without incredibly precise control of all the other factors - developer condition, temperature, film type, nature of images, etc. - there is probably no way of definitively evaluating and measuring that effect.
My impression - based on decades of using various types and sizes of tanks - is that, given consistent patterns and frequency of agitation, the difference is minimal between developing one roll in a one reel tank with 300 ml of developer (for example) and developing one roll in a two reel tank with some form of spacer and 300 ml of developer. I have lots of reels, so I use them as the spacer.
My Paterson tanks are almost all the latest version Super System 4, which do not come with a locking ring. I don't know whether the older locking rings would work with them - the centre cores seem to have different surfaces. If you can't use an extra reel, I would suggest a stretched and doubled over rubber band.
In my case, In recent years I've been using developer in a replenishment regime. As a result, I am almost always using a 3x135/2x120 Paterson tank, filling it with one litre of developer and putting in what ever number of filled and empty reels as is required. With replenishment, you don't end up tossing away unused developer, because it goes back into the bottle after you use it (less a little bit that is replaced with fresh replenisher).
If I were the OP, I would be more concerned with ensuring that my agitation patterns and frequency were consistent.
