Something like this?
Got a JBL K-140 on ebay. I found the plans for a Sunn 2000s cab like I used to have. Cut it in half for 1-15". and went to town.
Cost was for a sheet of 1/2" sandply $25. Screws, glue, grill cover, casters and jack w/ plate. I used shellac on it to give it a retro blonde look.
One of my tricks to building cabs is to cut a 1" strip and glue it to the front / inside. Then glue the face to it. Squares up the cabinet. I use 1" strips of wood for bracing and the back bracing for the back cover.
Look around for the size of the cab your wanting to build. Get the specs and your off.