A standard EXPRESS Rack single locker payload is 18.125" external width, with an internal width of 17.340". So COTS rack mount hardware would not fit facing the front. I suppose you could mount it sideways, since the available depth is 20.195".
Image Source (PDF)
If you really needed more width, there have been some double locker payloads that are a single unit high and two units wide. That's what HP did with their Spaceborne Computer payload, which
contains two servers and no modifications to its hardware. A
water-cooled enclosure will further help it deflect radiation while
the software will have to meet Nasa’s strict standards, in place to
protect spacecrafts and astronauts.
Speaking of Spaceborne Computer, you'll need some form of radiation hardening, either software (like SC is demonstrating) or hardware.
However, the size isn't the biggest issue with flying COTS hardware. All powered payloads must have some form of smoke detection. Payloads in an EXPRESS rack must either have air circulating through a closed volume that exhausts through the rack smoke detector, or if they're water cooled they need their own internal ECW (Emergency, Caution, Warning) enunciation that notifies the rack that the hardware is overheating.
The EXPRESS Rack supplies 28V DC power and an ethernet connection through the front of the payload. If your rack mount hardware isn't compatible with 28V, you'll need a power supply to convert the voltage too.