Hi, Stargazer! Congratulations on your new optical tube! There are plenty of plans for Dobsonians on the web, but finding one that exactly suits your particular tube may be challenging. It's probably easier to create your own plans; you already have most of the essential measurements. If you could take a few photos of your tube and post them here, that will make things easier for us as we help you along.
One of your first decisions should be whether to cut out the sides of the rocker box around the altitude bearings, or to have the sides enclose the bearings. Cutting away the sides will allow the tube to shift sideways in the mount until it rubs, unless your bearings have shoulders to prevent this. Go to Orion's website (telescope.com), zoom in on the XT6's bearing and you'll see the shoulder clearly. If your bearings don't have such shoulders, you can add them, or you can enclose the bearings inside the box as on the XT6 Plus. Which way you go will determine the width of the box, so give that some thought right away.
Think, too, about whether you want to make a plain box (the classic Dobsonian) or something more styled, like the Orion mounts. Do you have a table saw, or will you be working mostly with hand tools? Also consider what type of material you'd like to use, because this will affect the design to some degree. Softwood plywood from your lumberyard is inexpensive and strong enough, but it's hard to finish attractively. Baltic birch (hardwood with many thin plies) is a favorite: stronger, denser, and much nicer looking, but of course more expensive. It looks good with a clear finish or stained. Finally, think about whether you'd rather sit or stand to use the 'scope, as that will determine the optimal height for the eyepiece.
I made a Dobsonian mount for my Orion 8" f/5 optical tube. The tube measures 9.25" diameter by 38" long, and about 15.75" from the altitude axis (center of the bearing) to the bottom of the tube. My finished box is 12" square (outside dimensions) by 24" tall (again to the altitude axis) and turns on a triangular plate with the feet spaced 16.5" apart. I built it a little taller than need be to put the eyepiece at a comfortable height for viewing from a folding chair. Depending on your observing preferences, yours will probably end up fairly similar in size. I left the optical tube slightly bottom-heavy and use a magnetic counterweight at the upper end to balance it with my lighter eyepieces.