I opted for a parallel tubing setup. Most Pedersen designs I have seen, the original Dursley Pedersen, or those from Jesper Solling or from Kemper, use in general an angle in the seat tubes, for one reason or the other.
See the following Dursley Pedersen, for example, and how the seat tubes form an angle.
For my frame I wanted a parallel tubing construction, as it gives me much more freedom for the seat height adjusting mechanism. The lower part is made of two pieces of 12mm tubing, with two traverse tubes to reinforce. Shown here just after brazing.
The upper adjustable part of the structure is made out of 10mm stainless steel tubing. Ideally, they should fit into the 12mm tubing of the lower part. This, however, proved not true, and I had to find a way to decrease slightly the diameter of the inner tubing to get a proper fit.
This was not to be an easy task, as we are dealing with very slender and long (about 40 cm) tubes of inox steel. Finally, I came up with the following solution. I took a Pultra 10mm watchmaker's lathe that you see below. Added an extra D-shaped bed and a cross slide from a Lorch-Schmidt 6mm lathe, to produce an extra-long lathe for the purpose. Since I do not have a self-centering chuck for the Pultra 10, I used a chuck with adjusting bolts. That is why you see a dial indicator in the picture for centering the workpiece.
Not that I specially recommend this setup, but it worked for me, lacking a better solution.
Mitering the tubes for brazing.
Preassembly to braze in place the upper part of the trombone. Parallelism is the key.
Perfect parallelism and easy sliding action achieved.
Next, the securing bolts to adjust to desired length.
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