Basically, it is 25" at the widest point and 22" at the narrowest point, 12" front to back and 13" at the sides where the speakers are, and about 12" deep (but since the lid curves to the rear of the bike, it drops down to 7" at the latch.
I have some pictures if you are interested.
Joe
The one I'm looking at is a Mutazu, looks about like a HD Tourpak, and is 22.83" wide, just over 17" front to back, and 13" high. After measuring the I/S trunk mount it should fit fine. And, it's a heck of a lot cheaper than trying to find an I/S trunk.