This is not the first time I have seen this issue. I think the Marstrom boom is from a Spar builder. I did a quick check through work and did not find any tubing in your size with a wall less than 1/8. All the tubing I saw (6061, 6063, and 2000 series) appears to be from 1 supplier. My source had an interesting comment. He said they would start looking at surplus suppliers, MARINE SUPPLIERs, and the last the MARINE MAKERS in Austrailia and France. I checked and Sparcraft has some rectangular'ish sections stock. I know this does help when you are trying to keep the price down.