Sorry BK meaning the4 »Bike Freiday extended post which allows their frame to fold and have a long seat mast. It is a very long extention compared to the odd bike one sees that is `cool`and doesn`t have a seat post just an extension of the seat tube. Not familiar with your project, but I can say that Aluminum often ends up about twice the wall thickness of steel, so if that were part of your problem you could reduce accordingly. Of course these are crude eztimates, but then we don`t have an infinite range of tubing dimensions to sellect from. Oc course, if one is using all he different tensile strengths it thickens the choice a lot...