Go Wal Mart or any store that carries bulbs and get the little 40 watt bulbs with the little screw in ends. I like the round bulbs for my knobs. Take needle nose pliers and break off the screw in end of the bulb and pull out the filament. Catalize some hot coat resin and pour a little into 2 cups and mix with the colors of your choice. Fill the bulb half way with clear catalized hot coat resin. Kick it slow. Now pour a little of each color in the bulb and add a little more clear then more color then clear until the bulb is almost full.Now take a paper towel and put it over the hole and start spinning the bulb slowly the air bubble from the top will start mixing the colors as you rotate the bulb. when you get your desired swirls place the bulb on top of a paper cup and top off the resin so the bulb is full. Let it cure and it will shrink and rattle inside the glass. Break the glass carefully and sand the bottom flat. Use 6 didfferent size drill bits from small to a little bigger than the bolt you will install. Don’t worry about drilling straight the knob feels better on an agle when installed. I use 1 1/4" long bolts so most is inside the knob for strength. Epoxy the bolt inside the knob careful not to get it on the outside of the knob. Let it cure and you are done, the resin should be shiny and smooth on the knob once you remove it from the glass and no need to polish
Tom