It may be irrelevant to FlexATX X10SDVs but FYI my 8C-TLN4F based 1U 5018D-FNT4s came with one (ducted) fan connected to the FAN1 header, right next to the 24-pin ATX power plug.
EDIT:
@PigLover and
@Quasduco make a great point about ducting being good enough here, especially with the more defined front-to-back airflow in your (literal) case. As long as you're happy enough with your handiwork so the OCD doesn't kick in!
Full disclosure... the one Xeon-D system I added a CPU fan is a passive/fanless X10SDV-4C-TLN2F, installed in a Silverstone DS380. My other 3 are the 1U systems mentioned above and they seem fine with respect to temps.