I see two options:
Out of the two options, the Tesla extension cord is probably the "safest". By that I mean that the thermal sensor that's built into the NEMA 10-30 adapter for the Mobile Connector is where it's supposed to be; as close as possible to the 10-30 outlet. The AC extension cord is of course the cheaper solution but that also means a possibly lower charge rate, due to the voltage drop that may result in the extension, and as mentioned earlier, the relocation of the thermal sensor away from the source outlet. If you do go the route of the AC extension be sure NOT to coil up the cord; stretch it out. This lowers the possibility of it heating up more than "normal".
I don't know of a third party EVSE solution but there has to be one out there. Is it not possible to re-locate the 10-30 outlet closer to the vehicle?