You should have no problem with port length with a box volume this large. Using the Wedge enclosure 2 volume calculator, if you're box material is 3/4" MDF and taking out 0.09 ft^3 for driver displacement, the net internal volume is approx. 2.9 ft^3. If this box has bracing, deduct those displacements.
Here's a calculator to get you started. You can find what the minimum square inch opening of the slot will have to be by filling in the top calculator. I'm guessing about 37 to 40 sq in. After you find that, design your slot, giving it a width and a height.
This is where I can't go further because I don't know if you have the driver mounted on the slanted (hypotenuse) side of the box or not. Whatever side it faces on, I would suggest building the slot port along the bottom of the enclosure where you have plenty of room to work with.
With the width being 26", and two side walls of 3/4", the measurement inside width would be 24.5". If you were to use that entire width you could use a rather narrow slot...but that calculator will pop up a warning box that tells you that you should keep the slot ratio less than 8:1. Work with that and design the slot dimensions. Put all your data into the second calculator and it will give you the length to build the port.
After you do all that, and this is important, you have to figure out the displacement of the port structure (the walls and airspace that the port steals from the box volume). Deduct that displacement from the 2.9 (or whatever you had figured) cubic feet net volume that you originally entered into the calcuator, and do it over again. You should find that with a net volume this large for one woofer, a tuning freq of 40 Hz, and slot opening of about 40 square inches, that the port length should fit easily along the 23" bottom of that box with plenty of room to spare.
Here's one slot design that I figured, just as an example: You build a slot opening centered on the bottom of the box that has an internal opening size of 16" wide and 2.5" high. You make it out of the same 3/4" MDF that you built the box with. To find displacement, you measure that whole port structure, which will have a 3/4" wall on each side and a 3/4" top, times the length that the program tells you.
The calculator gives you the length based on the box volume of 2.9, which comes to about 10". You find that volume (you can use the rectangle box calculator on this site) and find that it displaces .3 ft^3. Deduct .3 from the 2.9 and re-enter 2.6 in the calculator. Now, when you press calculate you will see that the port length has grown because the box air volume dedicated for the sub has gotten smaller. Now it's 12.7" long and you have to figure port displacement over again. Make another adjustment to the box volume, and do it again. You have to do this over a few times until you nail it, or quit!
In this example, I finally came up with a 16" X 2.5" slot opening along the bottom of the box and a port length of 14.25". The air volume that is left for the subwoofer is 2.45 ft^3. This is a tuning freq of 40 Hz. The port structure is built with 3/4" MDF and centered, with it's own top and side walls. There is 7" of space at the end of the port inside of the box, which is plenty.
Give us feedback on what you come up with.
Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.