Eric, Mark's right - when I bought 527 back in '05 it had a carbon boom that was 'self adjusting' - unfortunately it was the wrong way round in that the harder it blew, the fuller the foot of the main became.

IIRC it was about a 45mm dia. I replaced it with an ali boom made from the broken topmast of a Topaz and, to be fair, there was bugger all difference in weight once it was fitted out. If you want to go with internal blocks, go with what ACE suggests - an ali boom large enough to accommodate the system you want. If you go for a round section you'll need to devise a way to prevent the blocks twisting inside the tube.