Nice if you have the copper and fittings already but for anyone that would have to buy the pipe and fittings the cost would be higher (and not any stronger) than other materials I think. I think you could make a boom arm just as strong out of one 10 foot length of 1-1/4" EMT conduit. It would be completely smooth inside so nothing for small bits of wood or dust to build up on, lower cost, and you could paint it with a quick coat of spray to match the Festool gray of the vac if you choose to do so. Also, 1-1/4" (nominal size, actual Id is ~1.38" or ~35mm) is about the same size as the hose. You could use it as part of your vacuum and attach a hose on the end of the boom and make a short connector hose from the vac or dust separator to the bottom of the boom arm. You could also use 1-1/2" EMT, the difference in price is minimal.