Epoxy fillers are incredibly handy, as they let you dial in the desired level of thickness — ketchup consistency, mayonnaise consistency, and peanut butter consistency.
There are a wide variety of commercial epoxy fillers available to professionals. However, hobbyists often use at-home replacements, and any fine powder can work. Things I've heard suggested — baby powder / talc, flour, cornstarch, tapioca flour, the fine sawdust that's produced by an orbital sander, any solid that's dry and can be processed by a coffee grinder, and graphite.
Products the professionals often use — fumed silica, microballoons / microspheres, thixotropic additives, and wood flour (which is just finely-ground sawdust).
(TODO — research) It's possible that cooling the two components before they're combined could increase the viscosity as well, since heating them decreases viscosity, per below?
decreased viscosity
Adding lacquer thinner or denatured alcohol to epoxy will thin it out. [1][2]
Heating the two components before combining them also decreases the viscosity. However, I heard there's a limit, after which overheating decreases the working time significantly. (TODO — expand)