Michael suggests:
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If you make a couple of "towers" to hold the straps, then this is easy.
Bolt the towers to the crosspiece (two bolts will do it - sloping the
towers to the outside will allow for more length to distribute load).
The towers could be 2x2 or 2x4 and only need to be tall enough to clear
the hull from anything underneath. Round the tops of the towers and
run the webbing over to the outside. Double (or triple) the webbing
by folding it over and use a hot nail to burn a couple of holes in
it. When you cut the webbing, use a lighter or match to burn the
cut end and melt the webbing so it won't fray.
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As always, much obliged!

Cheers,
Wilf