I cut the plate to the same max size of the tower and then the hole I cut in the tower is about 10mm smaller all round.
I did this so I could fit the plate from below and have the top of the plate touching the bottom of the tower.
That way the old tower stops the new plate pushing up.
Also so I could have 1 weld around the smaller hole on the top of new plate and another weld around 10mm away on the bottom of the plate. So there are 2 separate rings of weld holding it in place.

An experienced welder could have fitted the plate on top and made a nicer job but for an amateur like me, this seemed like the safest way to do it.