I have the parts inbound for a 50' 50A cord.
Ordered 50' of SOOW 6/4 for $187 on eBay. Really only need 6/3 with a fourth wire of #8, but couldn't find that. Found the Marinco 120/240 50A Female for $24, and a Marinco 50A Male (with pull handle) for $20.
So I'll have $231 in the project for a 50A extension/Power cord.
If I need to use this as an extension, I already have an adapter that I made to convert from Marinco 50A Female to standard RV 50A Female.