If you start from the bottom up it does not move espically if you have your other gear around it. There is no PVC waggling noise either that thing is solid as a rock. All the joints are glued together and you can use it as a ladder if you want to. My buddy was wondering the same thing this weekend when we were heading to the Dive Site so I put 4 tanks in it and tipped it about 60* and the tanks did not move. You would have to hit a corner pretty darn hard to get it to even start to tip over. When I do put 6 tanks on it I would put a bungee cord over the top ones but even without you would probably be OK.
It probably would not fit into a car. I do drive a SUV that I put it in, although if you only make it for 4 tanks it may fit into some car trunks.
Here is what I used to build it...
(9) 24" Sched 40 PVC pipe
(8) 10" Sched 40 PVC pipe
(12) 6" Sched 40 PVC pipe
(8) 4-way connectors (for bottom and middle corners)
(4) 3-way connectors (for top corners)
(6) T connectors
The cheapest place that I could find the 3-way and 4 way connectors was
www.usplastic.com
If you have a vehicle big enough to put one in then I would suggest building one. You will not be disapointed.
Enjoy
Phil