In my recent post about my mortgage overpayments I mentioned a few of my side-hustles that I use to earn a few extra pennies to go towards the mortgage.
One of my other side-hustles, although not dedicated to becoming mortgage free (it goes to my Lifetime ISA instead) is Zazzle. This print-on-demand company allows you to upload your original works, set them up to print on various products, and then list them for sale. Zazzle handles the printing and dispatch, so you only get a small amount of the money, but you can bump up your percentage (this does bump up the cost of the product though) to earn a bit more.