kraton (the ruber cold steel uses), tire rubber, and epoxy soaked paracord are not used by busse combat. kraton breaks apart in the hand after x amount of time, tire rubber smells aweful and isn't that comfortable (though you could always try, it might work out for you), and epoxy soaked paracord is impossible to get off if it ever cracks or gets messed up.
there are 2 types of busse customs, custom varients sold at shows, and custom orders. since busse combat has previously used reciprene c in their basic line, you may still be able to get it from them on a custom order. it is by far the grippiest of any of the handle materials. Busse combat has also previously done handle wraps, so you still may be able to get a pbf style handle wrap, but that would only be on a custom order, and they may not be willing or able to do it now.
with all of the other natural materials used in busse varients, ranging from horn and mp5 (or whatever the stealth hawk material is called), to alluminum (used on the ak's, i'm sure they could do one on a custom) and burl wood - unless it's got a pattern cut into it, it's not gonna be very grippy. most of the custom variants have smoothed out handles, rather then the rope pattern, the daily carry pattern (or whatever the sar4/5's is called), or the fusion patter. smoothed out g10, paper micarta, wood, horn, mp5 or what have you is all pretty solid materials with little texture. so having it smoothed out like that leaves a reletively slick surface that doesn't grip to skin.
The only material I've personally handled that isn't like that is roughed up canvas micarta. and thats only because the canvas threading in the micarta tends to fray out a bit and creat a rough surface texture. with paper micarta, the threads are so thin that even if they did fray out, you wouldn't feel them. But even with canvas micarta, when it's smoothed out like on the shba's, its still fairly smooth and non grippy.
so if you want something thats really "grippy" and solid in the hand on a pure custom, I'd say go with a custom pbf style wrap, reciprene c, or canvas micarta with a pattern cut into.The custom handle materials vary much more when it comes to durability and weather resistance then they do with grippyness and texture.