[TowerTalk] Capacity cage on low band vertical?

[Mike Bragassa, K5UO]

I am in the design/contemplating stage for a 80m and possibly 160m vertical.

I can put up probably 75-80 feet of Rohn 25g that I already have; using something like Phillystran for guys. Then I would put some kind (?) of tubular stinger on top of this.

What are some of my alternatives with this arrangement?
It likely appears that I will need some sort of capacity cage somewhere on the upper portions of the tower. 
How do you calculate the dimensions of this cage?
Then I would go with a tuner/matching arrangement at the base using relays to switch between bands.( In the coil or not in the coil.) 
I recall  a similar type of antenna that Array-Solutions sold that used wires folded back near the top of the antenna. They were tied of with rope out to the perimeter of the radials; say 70 feet.

Any suggestions passed and the time expended is very much appreciated.

Mike, K5UO