I have a Thule box on top of my Syncro westi so I can let you know from experience that...
Adding a box to the top of a pop top makes it really heavy to lift up with stuff in it.  You can add gas struts to the pop top to aid in lifting with a loaded box.   Also don't roll your van onto the Thule box as it is not built to hold the weight of the van on it!
Ask me how I know.
Brady

----- Original Message ----
From: Sean Hyde <seanmhyde@HOTMAIL.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Sunday, August 30, 2009 5:23:42 PM
Subject: Thule cargo box -- on pop-top or hitch?

With a long road trip on the horizon for our family, we are considering
adding a cargo box on top of our pop-top (Thule model "Ascent 1600"??) or
a rear-mounted hitch version (Thule model "Transporter - Combi"??), for
our 1991 Mulivan camper.  Does anyone have experience with either
scenario, and thoughts on the pros vs. cons of each?  Our concern with
mounting on the pop-top is the possible inability to raise the pop-top
with the cargo box containing belongings, as well as possible wear on the
pop-top hinges.  Any feedback you have would be appreciated.  Thanks!

Sean & "Winnie"
Seattle, WA