the Odessa X pattern turret is a quite simple scratchbuiding job... step 1: think a template (or at least steal one). personally i made both things, i've stolen one simplyfing it. step 2: build it: i needed only some glue and 1mm thick plasticard and this is the result!
writing the measures is a nice thing to do if you wanna avoid mistakes!
rear view of the diamond-shaped turret
another nice thing to do: try many ways of putting pieces of equipment on the turret with some adhesive putty
when you are happy with it, it's time to glue! bear in mind what you wanna paint (insignas, numbers...)
in order to have a nice rotation of the turret, the best and simpliest thing to do is make a cross with plasticard, obviously of the same dimension of the circle in the top part of the hull and fill the grooves of the hole (that hold the hooks of the original turret) with milliput.
after this, simply glue the gun one the turret and you're done!