That is what I figured you meant. Thank you for the picture. It looks as if you can utilize a round port for this without having to modify too much. Most of the port can go into the enclosure also. From very very rough calculations, I figured you can utilize a single 3" round port that will go into the current setup about 19" and get a response at least down to 34-35Hz with more authority than you have now. I say only around 34Hz due to port noise being introduce likely below that at max power. But below max power, around music level, you should be fine to around 30Hz. Max voltage is about 32V, and music voltage is about 22.4V (calculated).
I really hope this helps you out at least a little. So, if you make a hole to fit a 3" round port into it and make it about 19" you will get a better low end response. Not optimum, but better without a rebuild.