The thickness needed for threading in barbs would make the block very heavy if Cu was used, check out the wieght of a 6mm thick peice of Cu, a 'smidge' it ain't!!...
Lexan is very expensive, £150 for a 4'x8' sheet in the UK(and thats 5mm not the 9mm that's needed for threading!), flat ALu bar is the cheapest option, which is why it's used...
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