Lining the inside of the cabinet with fiberglass helps to reduce reverberations inside the cabinet and makes for a more focused sound, but sometimes also a sound that isn't as rich.
The fiberglass (foam on later models) is used to damp unwanted resonant frequencies, and assist with limiting the porting to low end reinforcement. Without some sort of damping material the tone would likely become "rich
" to the point of cacophony.
Use the depth and width dimensions of the head you're going to match the cabinet up with ( ie: 200S or 2000S) and, and use the drawing you have, and design the cabinet with those dimensions. Should work out good. To get the original sound, a couple JBL D140F's are necessary though.
The width part is correct, but both the 200S and 2000S cabinets are substantially greater in depth than their corresponding amplifiers.
The exterior dimensions are as follows;
200S - 42"H X 24"W X 15"D
2000S - 48"H X 30"W X 15"D
The porting is key. It's relatively easy to build a box the right size, a different thing altogether to get the porting
They don't come along too often, but it's probably cheaper to buy an existing 200S cab than it would be to build one. The last one I bought was $375 with
the original D140's still in it.