While it's usually not recommended to use an AC extension cord with a mobile connector, I find it just on this side of wholly unacceptable for somebody to construct a DIY extension cord for the Tesla Proprietary Connector. EVSEadapters.com has those extensions in their catalog, but as you say, they are out of stock for every one if the listed lengths they sell. I would contact them and enquire how many wires they have in their extensions cords if you insist on proceeding with this project.
It would be much safer and infinitely easier to accomplish the extension by using an AC cord. Find one which has a large enough gauge wire for whatever outlet you are using (14-50, 10-30, etc.). Those are easy enough to find on Amazon. You may be circumventing a safety circuit that is built into the Tesla NEMA adapter; there's a temperature sensor in those adapters which monitors the heat of the outlet. Using an extension cord places that sensor away from the source outlet.