Sorry I didn't get back to you yesterday as planned, husband took a bit to track down the link. He thinks it's this one:
https://blog.hemmings.com/index.php/2007/04/26/almost-free-garage-heat-just-drink-a-lot-of-soda/
He changed up the size, obviously, and used tempered glass instead of plexi because he gets it cheap at work, and he spray painted the entire thing in matte black. Also made the interior slightly larger to accomodate some foam insulation sheeting too. There are several other variations on the plans, like adding a small pc fan to the outlet at the top.