That's how i made mine, with silicone to glue them together, and used a safety vest (cut to shape) from bunnings, couple of U bolts, cheap as. Leave it off until you hit the desert.
God, I wish I was heading for the desert right now.
JC
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So 2.5m above a defender roofrack should be OK then :D:D:D:D
Mines been cable tied to the roof rack since our last trip in 2012
Martyn