I have a Royal 880 that I use at work, that has the original bag, designed for Royal Type A or Eureka F&G bags. Last week, I used up the last bag, and had some trouble finding more. So, an idea struck me - maybe I could convert it to use Kirby bags. The base of the fill tube on my Kirby Heritage II Legend looks just about the right size to slide around the A-bag fitting.
So, on that idea, I ordered a fill tube for a Kirby Generation-series. The modification is even easier than I thought it would be - just slide the retention spring up over the fill tube (it will crush the tube slightly), then slide the fill tube down over the bag fitting. Next, secure it with both the original spring and the Kirby-supplied zip tie. It fits on there nice and snug, and doesn't seem to leak any air. Then, just install a Kirby Legend/Generation bag as usual (preferably one of the new synthetic bags), and voila, you have a bottom-fill Royal converted to top-fill, with better filtration and airflow to boot.
The only problem I encountered is that there's nothing to tie off the top of the fill tube, so it will flop around when the bag is removed. This doesn't seem to cause any problems, but you could attach it to the bag chain with a zip tie or a shoelace if this really bothers you.
The great part about this is that there are no permanent changes. If you want to go back to the original configuration, just cut the zip tie and pull the fill tube off.
I don't know if this would work on Sanitaires, Eurekas and other F&G bag vacuums, though I suspect it would if their fill tubes have the same kind of flange at the top that the Royal one does.