Tower

[K7SGJ]

I have a sneaking hunch that your tower may not have a 'top section' ... such as one of these:
Rohn 25AG1.jpg
So, if that is the case, you have two choices:

(1) Acquire a top section
(2) Get a flat plate with a thrust bearing in it to mount across the top.

Do you know what make/model the tower is?

My first look at that, and all I could think of was putting a few Jetex engines on the legs.