Chris and others interested in the roof-top AC unit (small window-style),
I have the setup documented on my website that is similar to what Chris is
talking about.   I usually take mine down and put it in the van when I
drive, but you could leave it up there without any problems if it were
secured down properly.   Take a look at my setup under "upgrades" on my
website.
Matt
'87 Westy/'90 Subie 2.2L

website:  http://home.earthlink.net/~subaruvanagon
"Life is a journey, explore each day and treasure every minute given to you"