From the pictures I've seen they look like SO Cord Connectors. Most pit builders use them. They come in 1/2" and 3/4" NPT fittings with various internal sizes to fit different size cords or thermo leads.
You can find them at Lowes, Home Depot, and all electrical supply houses.
Weld a pipe coupling of the right size to the pit and screw the SO Connector into the coupling. You can even cut the coupling in half and make two weld in's.