We mount the APU in a standard 19" 1HU rack, without any special solution. We put the flat heat sink metal plate on the bottom wall and 5mm spacers for the APU to fit it at the right disance from the floor. We never had problems, I suppose that with APU2 it can be the same, it consumes a bit less.
Yes, using passive cooling. Because board is mounted in plastic-enclosure with some airspace within, and venting holes in enclosure, CPU "pressed" to (large) metal plate, providing heat dispersion. Might need testing for stable ops in hot surrounding, though.