Aaron 2 solid steel rollers flatland gap adjustable for gap. Crank handle on each and pore shot in gap while tuning. As you and a helper turn shot will fall through untouched while others are deformed. Trial and error until about half or less are deformed. A third hand to pore shot is handy.
I have found the X spreader to work well in 10 and 12. Use the flattened in 20. My sons were just starting out when we started using these and it doubled their hits. Now they consider it cheating although their bottom barrel contains a cyl. choke.
Younger one made it as a project in a welding class in trade school.

I awlays found the Polywad SpreadR loads to be the best I ever shot. I still have almost a case of them which I save for my daughter to use in an essentially cylinder bore RBL 28 ga. It is .002 and .008 in the 30" barrels. These loads make the left barrel shoot about 20% more open.

I have always found the plastic x type wad much better than cardboard. I buy into the theory that it compresses in the forcing cone and choke and then springs outward to spead the shot. I have always had much better luck than with the cardboard ones.

When I am out of the PolyWad loads, I will use a plastic X over a plastic gas seal/overshot wad with no shot wrapper. I believe this will give the best radial dispersion. Shot will be soft lead or low quality.

Art I also own a 28 gauge RBL only with 26” barrels.
Have always shot RST Spreaders through it. Wonderful patterning gun for close flushing Mearns quail over my dogs.
Im getting to the end of my RST supply and due to difficulty in obtaining the spreader loads as well as price I am thinking of getting back into reloading, at least for my 28 gauges.

Rodger Barlow wrote an article on cubic and disc shot in The Gun Digest annual in 1973 edition.These can be had in digital form or from various booksellers.