I scan all sheet film, regardless of size, directly on the glass using a wet mount system. For my scanner, I get the sharpest resolution that way. Using any form of mount reduces resolution because it takes the film away from the plane of focus. Each scanner is different, however. So you have to test your own scanner to see where it works best.
One thing you might consider is using the negative carrier as the film holder for scanning. I've done that as well, but only for DSLR scanning. Still, I don't see why it wouldn't work for a scanner, so long as you left the bottom part open so the scanner can calibrate itself during exposure. You could also build your own DIY scanning holder using black card stock. Get it in the appropriate thickness, and then cut out a pair of holes to align up with the edges of the negative. Then tape them together as if the tape was a hinge using some strong tape (I use Gorilla tape for this).