You have an exhaust fan up top. Drill or cut out a hole in the top.

Seal up the rest of the rack. Side panels and a rear access panel or a rear door (not sure whats available for slim5).

Seal up the front with RSH shelves.

Front bottom of the rack use a 2U vented panel or front rack fan.

If budget wont cover this just cut the top out and leave the sides and back open. Looks like plenty of room in there so you might be okay. Hopefully that room doesn't turn into a junk drawer with crap stacked allover it.