Originally Posted by CAW2018SiestaSS
that's the problem. I've no idea where this thing terminates and haven't found anything.
This is not as daunting as it seems.
Grab a tape measure, and measure how far from the rear corner that hole is. Then measure how far it is inside the bay. Finally: measure down from the bottom edge of a window to get an idea of the height...
Once you're inside: take about 3 inches off of the distance to the rear corner (for the width of the wall and insulation... a fair estimate), and maybe an inch or so for sidewall thickness...
This'll get you in the ballpark: happy hunting!