Both solutions offer benefits while I tend to lean towards teleprog's suggestion... the new acs design does not include a top cover, with a simple backplane much like the older ones with the cover removed.
Otherwise, as long as your "fan-forced" switch is in a location where the fan operation can be monitored regularly as a by-product of simply walking by, etc, any heat removal is your friend.
My only concerns releated to strapping some fans on top is that regardless of their incredible life, they are subject to failure, and in their non-operational state, the extra frames and non-moving fins will only add to restricting heat.
The second concern is in pulling unclean air through. Using natural convective currents, most dust/lint/debris is too heavy to become pulled into the circuit packs. When you start to use fans to force airflow, be careful to make certain the air flow vents to not become blocked. If you've ever pulled apart a PC computer powersupply (or fan forced case) you'll see how heavily caked the components, grills and blades all become. The finer screens covering the heat vent openings may act like a plugged lint screen on a clothes dryer.
In short, I realize may of our end-users want to place KSU's into service in a variety of "harsh" environments including damp basements, attics, non-climate controlled closets and rooms, etc. Many of these customers are small system "old school" customers that often are replacing old switches 1A2, electro mechanical, and early electronics which didn't have microprocessors or hard drives. The mentality is often, "it was a good enough location for Ma Bell to install the old one."
Gently remind these end users that we're no longer talking about circuitry comparable to a garage door opener, but rather highly sensitive with all the electronics of that desktop computer. Demonstrate their own care of these desktop devices noting any surge protection, practices of unplug during lightning storms, cover at night when not in use, etc. They'll be a lot more receptive to your ideas regarding location including environmental and power conditioning suggestions.
