I made a simple wooden frame to mount my amp in, got some 45 degree rca connectors and it fits on my gear rack. A heck of a lot easier than trying to mount it into the tube and then try to seal it.
Also there is a debatable idea that the vibrations from the sub will actually make the amp...
I just got done with my third shiva sonotube enclosure. You can't go wrong with sealed.
The first was a 12" tube 48" high, about 92 liters.
The second was a 12" tube 25" on its side - front firing - heavily stuffed, at about 50 liters.
The third was an 18" tube 39" high, about 142 liters...