The diameter is 45 or 50 mm and the wall thick is 2.5mm, i think is steel.
The D loop is 3 or 4 fingers low my shoulder and is ok. This measure is like my other car (Mazda MX5) with a D loop on a same place, more higher and I think can be dangerous.
Also I lining the tube bar with black air conditioning sponge and looks very good.
I search on ebay and I find the original seat belt bar, but normally are very expensive and also i believe don't support the hi-lift.
Link: http://cgi.ebay.co.uk/Seat-Belt-Mounting-f...Q2em118Q2el1247