Thanks Martin and Mike
Neville. This pic is what im doing, pretty rough and not accurately to scale. The PVC angle and bar used are pretty thin and flexible, but stiffen up when bent with a heat gun. I used a rubber strip to pack out the dome ring diameter, I made the ring too small, it should have overhung the base ring, but most of this build design was on the fly as I went.
I cant really use rubber on anything that shows externally. Bunnings only has black, this whole thing needs to be white (to match house,daughters order). I tried to paint black rubber white, too flexible, didnt go well.
I haven't shown flashing tape on the pic, it and sealant will be applied where required.
I could have just made one large skirt over everything, but im trying to get the interface between the base and dome to look as slim and non-clunky as possible.
I thought about flared Buyte flashing tape, its used on the segment joins and the slot cover, but it does look a bit loosy goosy, I thought id try a bit harder on the base.