you've got a few options with the DN2800MT.
you could use the on board power and do some sort of 12V regulation outside the case
you could also use the on-board "P2" (it's half of a typical P4 connector) and provide power to it with any of the ituner psu's (or other automotive psu's)
one option would be a DCDC-USB from ituner; you can use it's p4 output and the p4 cable will plug right into the "p2" on the DN2800MT. The extra 2 pins will just hang off the edge.