Late Swamp People Star's Co-Star To Finish His Dream Project

The man behind the promise is Glenn Guist -- Mitchell's bearded brother and best friend -- who is often featured hunting critters with Mitchell on "Swamp People."

Glenn's manager tells TMZ ... he's pretty broken up about the sudden unexpected death of his brother ... but he's doing better then they all expected, considering how close they were.

We're told Glenn knew how passionate Mitchell was about the houseboat project ... and believes completing the project will be the best way to honor his brother's memory.

As we previously reported, Mitchell passed away Monday after apparently suffering a seizure while on the bayou in Louisiana.