I had a 6" to 5" reducer as the first peice inside my bed, so I used a 5" holesaw for a nice clean cut and made a notch for the clamp with my sawzall. This made for a nice tight fit at the bottom of the stack.
For the top, you can get a SS stack mounting clamp at any bigrig/chrome shop. Some people don't like the fact that you can see it at the top of the bed, but I do. I think it looks good, and it is as solid as ever.
These are old pictures before I lowered the stack, but you get the idea.
Let me know if you want one, and can't find one. They are like $20 I think. They have them down the street from me in all sizes.