Actually Dylan its too bad it is not mine or here so I could just do the measuring and give some exact numbers.
I could come up with some close approxamations but would prefer someone in the rebuild process to take some real measurements.

Cabinets are 30" wide so interior is 28.5.
They are 20" deep so interior is 18.5
I don't have the seperate box heights right here and also have to consider the black seperator between both as well as the horn mounting face being inset some.

I think on the site that Grumpy provided above by Jeff Markwart the numbers are there but it takes so freaking long to get anywhere from here with dial up.
I am also at a bit of a loss in determining how much interior space to subtract for bracing and component intrusion.

Gary