I think a 40 inch long piece of 6 inch diameter PVC would work well. Having the open end near the floor should not be a big problem, you might try using a 12 inch square wood base and mounting the pipe 3 or 4 inches above the base on attached legs. I have seen similar things done many times without any hints of poor performance resulting.

I remember seeing a sub done this way on the DIY Forum, I think. Basically it was something like a 12 inch diameter tube with the driver at one end near the floor. The sond came up this tube and then wnet back down a 15 inch tube mounted concentric ans paced about 3 or 4 inches above the end of the inner tube. The sound then exited the outer annulus near the floor. This arrangement was a different way of folding the line in half.

I am also waiting to see the next alpha TL article from Schultz. I have communicated with him a few times and we have swaped ideas. He approaches TL design and alignment a little different so it is always interesting hearing about his results.

Martin