I mounted mine outside the bed between the bed and the cab. Simple "L" shaped bracket did it. Was a PITA to install because of the lack of space.
Suggestion, mount the antenna to the bracket first. Then install the nuts, bolts through the bed. A friend in the bed using a ratchet while you
hold the nut with a box would probably make things easier. I mounted the antenna that way because I have a hard tonneau cover on my truck.
If you don't and want to mount the bracket inside the bed that should make it easier.
This is the only decent photo I have of it.