In my Sidleock 50 cal and both my .32 inlines I used Pillow Ticking. You can get it by the yard at Some Walmart's but I go to JoAnn Fabric. JoAnn near me had 3 different Bolts of It, one was red strip and the other blue and those felt thicker than one of the other blue striped bolts they had. I use the thick one and .315 Hornady RB. I buy a yard at a time and then wash the material whole, dry it, wash again and then tumble it mostly dry then hang it. It helps Fluff it up and removes the sizing treatment. You can buy PreCut Pillow tick and even get it prelubed. I use Hoppes #9Plus for my lube, dampen a section of ticking and roll that up and sort of squeeze and wring it so it is just damp. I have never used Bore butter so can't comment there.
To patch a ball, Cut or rip the ticking in strips wide enough to cover the width of the muzzle. Place a strip on the muzzle and lay the ball on the ticking over the bore, Short start it just below the crown, then pinch up the ticking away from the muzzle and take your knife and cut it off across the muzzles crown. Perfectly patched ball every time. This is how and old timer taught me many moons ago and so its how I keep doing it. Works great for me.
The Marathon is a little tight with the .315 and the ticking I use. I first tried .310 in it but one range session I had run out of .310's so tried some of the .315's and they shot bout the same or A little better most likely because they loaded a bit tighter. I like to have them tight enough that you have to start them with a short starter. I have used Pyrodex P but have shot T73F too and both work very well. I swab between shots with a little spritz of Windex on a cotton patch. Youcan pick up a small spray bottle of Cleaner for eyeglasses at Walmart or drug store, use that till its empty and then use Windex. The Small bottles fit in your pouch or pocket easily.
For Round balls sizes the rifle will have to tell you what it likes. The combination of Ball diameter and patch thickness have to work together for the right fit. I mostly see .310 and .315 for sale online from popular retailers. You can also get molds made too if you are into casting. One day I plan to get a Moose mold in .315.
I'll be watching for your report on the Crockett, I have thought about getting one too. Never really be a fan of Tradition rifles but that one is one I do often think about looking into.
Have you bought any RB yet?