i did nothing special to my dreamcast or the 12v pins. i just took out the drive, put in the gdemu and closed it up. i was worried about heat also when i got it but even with the stock fan it seems just fine, just dont block your vents. i ordered a 3d printed tray but it wasnt even the right size because i had a clone and something was just a little off, if you just want something to look nice id go with one of the extender cables and a tray that sits up higher flush with the dc shell, though i havent tried one.
- edit - the tray even if its a recessed one would be nice to have simply in the event you drop your sd card when trying to put it in or take it out. it has happened to me and i had to fish around to find it but so far it hasnt been much of a problem