is the RM display going in the console in front of the helm or up top in the overhead elect. box? if going in the helm console, try using HD marine velcro and "stick" the module inside the deep console at the helm which makes the cable run to the display at the helm very easy. It's been a few years since i installed my C80 on a former boat, but flat mounting the DSM should be OK. Velco enable you to remove the module if the need ever arises and eliminates the need to make hole with screws in this area. Try to mount so that you can view the diagnostic LED should you need to trouble shoot. One time my C80 was blinking that the xducer was not attached and a yellow/amber light on the DSM was flashing. A rat had chewed the xducer cable right above the xducer so i couldn't fix it but had to replace xducer :evil:
If flat mounting hides the LED, just have a small mirror handy to check the LED (use some more velco to keep it handy)
Overall, the DSM install is very easy (ground connection, DC power, Xducer cable in, FF cable to C80)