I really liked QuickbeamX's spell slot tokens (https://www.thingiverse.com/thing:2385548), but they seem a little large and require supports.
I decided to make my own, and a handy container to keep them organized in. Aside from levels 1 and 2, each token has a number of sides equal to it's level.... so no need to worry about 6th and 9th confusion, plus they each have their own look.
I will probably revise the tray, it's functional, but not as clean as I'd like. I may make a lid or something for transport and maybe some dragon or d20 logos.. feel free to modify or make your own.