I like Martingale collars, if you want to make a rope martingale, the easiest way is to buy the martingale chain, and just loop and clamp rope through on both of the loops.
If you want to make regular collars, meaning not rope, you can order the belt material here:
Webbing for Purses, Bags, Backpacks, Leashes and More!
and buy the sizing hooks, and clasps at the last site I linked. I think the rope might not distribute weight properly, and the belt will.
You can make a super cheap choker leash by putting a loop on one end of the rope, and making a loop of the rope on the other end for a handle, and just slipping the middle through the metal loop on one end making a choker, but you need to tie a knot in the rope towards the end, or when in a relaxed state the rope expands big enough to fall off the dog's neck.