Looks like the "no slot" design has won. And for good reason: it's the most flexible option. I'll include a spring with each order so you can convert the knob to the slotted design if it works loose.

Thanks again for all your input and ideas. I am submitting my approval to the casters. Now we enter the LONG wait as the metal mold ("tool") is created. Next step: a few clear sets as production verification samples!! Early March? We'll see.