Calculate the dimensions with the XLS file here.
Put an N connector to it.
Put aluminium tape to connect the boxes.
Put it in a plastic bag. Or even PVC tube.
Fiber glass mast at 5EUR.
See my other prototype made out of printing offset aluminium sheets:
A french radio-amateur is putting the tape in the middle of the guide, where the impact on the performance is minimal:
Check also this one made in Thailand (64 slots, 27 dBi), crazzy stuff: