You say it is a very small form factor, but exactly what? :-)

o Micro-ATX - CPU can be P3-Cel (30W) to P4-Prescott (115W)
o Mini-ITX - CPU can be VIA-C3 (12W) to P4-Prescott (115W)

You say it piles out a lot of hot air, which makes me think VIA C3.

Duct/Extractor to Outside = Static-Resistance + Continuous-Rating + Noise-Level
o PC-style 12V Axial fan - eg, 120x38, 171x51mm
---- +ve -- Continuous-Rated + Quietish + Cheap
---- -ve -- Low Pressure even from 38-51mm depth fans = Low Airflow
------- you can get away with a 172x51mm eg, Papst 6212NM, but it's not quiet
------- so you are then into noise-proofing the cupboard &/or fan
o Extractor Axial fan - eg, shower, toilet etc
---- +ve -- Easy Build-In off the shelf fitment + Acoustic enclosures for loft use
---- -ve -- NOT always Continuously-Rated (some Vent-Axia are) + Not quiet + Cost

Since this is just a single PC, I don't think this solution is worth the hassle.
With 16x 1U dual-processor PCs doing CFD it is a different matter re 4kW of heat.

Noise Reduction:
o Distance -- dB(A) drops off rapidly with distance
---- use K-V-M cable extensions to relocate the SFF-PC further away
---- the catch-22 is your removeable media - altho external USB enclosures exist
o Noise Source -- fans are one source (quiet fans exist), HDs another source
---- targetting the noise source is more effective than treating the effect
o Noise Effect -- an enclosure (cupboard) can be useful for reducing noise
---- soundproofing must use Mass (low frequency) & Absorption (high frequency)
-------- TreadAir Carpet Underlay - rubber (Mass) rippled foam (Closed-Cell)
-------- carpet may include nylon - running the risk of static
---- do not fit ventillation fans to the cupboard (re PC / soundproofing / fan / ear)
-------- soundproofing should between the source & your ear :-)
---- every 90-degree turn reduces noise by 3dB(A)
-------- like distance, acoustic baffles work well - especially if lined

Cupboard cooling:
o Natural convection
---- wood-slatted-door (prettier than vents)
---- at the rear of the slats, close of all except for the top & bottom
o Forced cooling
---- PC Axial fans will be ok here
---- Do not fit to cupboard front (line intake path to absorb fan noise)
---- Do not fit in narrow-end of an enclosure (acoustically this is ugly)

Baseline before/after the design:
o Identify Thermal Baseline Before -- CPU & Board Temp + HD Temp
---- onboard monitoring tools (MBM) + S.M.A.R.T. HD temp checker (HDDTemp)
o Identify Thermal Baseline After -- objective is HD 50oC, CPU 55oC
---- some degree of thermal stressing re s/w - since ambient is 30oC summer

Airflow required:
o EBM-Papst figure is 300cfm cools 1500W before adjustment for resistance
---- a crude heuristic rule of thumb ignoring ambient & mass of air, etc etc
o Adjustment will come down to solution choice
---- lo adjustment for cupboard - room (120x38mm fan will lose 25% of free-air
airflow)
---- hi adjustment for house - outside (120x38mm fan will lose 80% of airflow)

SFF enclosu
o You are re-housing the machine in a cupboard for noise
---- primarily I suspect this is due to fan noise, secondarily due to HD noise
o So why not re-house the machine itself in a different enclosure
---- one using say 80mm fans or a 120mm fan - in place of 60mm fans
-------- an issue would be a high-end graphics card fan - passive solutions exist
---- an enclosure such as a flight-case - £30-50 on Ebay
-------- off the shelf gives you dense plywood, nice finish, castors, door labyrinth
seal
-------- www.penn-fabrication.com do all the parts if you need more
-------- easy to add perforated-dish vents & inset the fans, line with carpet underlay

The easiest is tackling the source & using distance.
The messiest is an enclosure with duct through a wall - soundproofing the duct, the
enclosure, the intake, and achieving a quiet fan which is continuously rated. As you
move into larger fans they move from electronic cutout to thermal cutout - that means
if they fail (locked-rotor) or jam they will get extremely hot indeed re fire risk. That
fire
risk exists for cheap shower extractor fans which are not continuously rated, even more
so if you enclose them in the loft acoustic boxes - typical is 15min on, 30min off IIRC.

I would use distance + recycle an Ebay flight case + carpet underlay.
If a VIA C3, a small bottom intake vent & top exhaust vent in a cupboard will do fine
(convection).

Keep It Simple :-)
--
Dorothy Bradbury