If your main concern is to secure the knot, I have found the best way, is to take a lighter and heat up the frayed ends of the knot so they begin to melt. If it starts to catch fire, quickly blow it out. Then quickly, before it hardens, take the side of a butter knife and squash the melted end down against the knot so it flatens like a mushroom. Take care not to burn the knot itself when melting the ends. After the ends cool, you are left with a hard little nub that will prevent the ends from slipping through. Remember to do one end at a time.
This method only works with nylon cord/boot laces. Regular rope or cord will just burn to ashes. But Nylon Cord will melt and then become hard like plastic after it cools. I have been using this method on cord chucks for 30 years and have never had a knot slip once in all that time!
No great ideas for the cosmetic issues. Only thing I can think of would be to use a metal crimp to fasten the two cord ends together. Years ago I purchased a pair of chucks that were fastened in this manner.
Hope this helps.
