Remove the enclose, seal the inside of the port and the mounting hole, fill it with water then measure the water. I haven't seen a volume published for it.
Personally I'd stick with a quality 6x9 due to the small volume (even before displacement) and lack of rigidity. There's also only 3" of mounting depth.
I was hoping to get a measurement before I ripped everything apart and then have to decide what to use with the panels apart, but your method will certainly work. If I do go that route, I'll report back on the volume. There certainly is a question about rigidity, but I will have to see what options I have there once I get in there. I know it is a small volume, but there are a good amount of small 6.5" subs out there that do not require much volume. Finding that actual mounting depth is a key parameter to limit my choices down. My ideal sub to put in there is a JL 6W3v3 as it only needs .15 ft^3 sealed, but the mounting depth is pretty deep at 4.25." Moving to something like the skar evl-65 drops the mouting depth to 3.5", but sealed box goes up to 0.25 ft^3. The shallowest option is the earthquake SWS 6.5X with 1.8125" mounting depth and 0.2-0.4 ft^3 sealed.
I like the dedicated sub instead of a 6x9 because I was going to completely remove the rear fill otherwise.
For the mounting depth, i recall from many years ago when i replaced the rear 6x9 speakers that i had to cut the port/tube that protrudes into the speaker enclosure because the magnet from the aftermarket speakers was much larger than stock (e.g., aftermarket speaker was "deeper"). This was a simple fix however - i just had to cut part of the plastic tube and the new speakers cleared without a problem
I think if the volume of the enclosure is small, I would have to remove the port all together. I doubt it has the volume to support a port of any kind as ported enclosures need more volume compared to their sealed counterparts. Cutting the port changes its characteristics also, so I again would probably just go sealed unless the volume and port characteristics happen to line up with a speaker I can find.
Thanks for the input!