if you have the room to dig into the hill, I would do that, but remember to keep a little slope so water will not run back into your feed. you stated that you feed 80% in short order. How long does the last 20% last? Well managed bunkers do not have any more loss than bags, and well managed bags do not have any more loss than bunkers. Both of them require 6 inches of face removal daily. When we talk about well managed bunkers it starts with packing. This can be an expensive process if the chopper is to big ( end up with poor pack & spoilage),or too small ( tractors running that are not doing anything), and then management ends with feed out. If a bunker were to last all year you are talking about needing a 180 foot long bunker. Packing density of sileage at 45# per square foot and a 7 foot average depth would require a 35 foot wide bunker. 7 foot of average depth would be 6 foot on each edge and 8 foot in the middle. 6 mil plastic will cost you  5-5.5 cents per square foot. Tire sidewalls to cover the plastic cover will cover about 12 square foot each and you would need about 525 of them. They each cost around 2.50 and will last forever. as far as reusing the plastic, darn near impossible as if you tried to roll it back as you fed it you would be creating a very nice rodent house and when it is cold and icy it is tough enough to remove 1 row of sidewalls and cut 3.5 feet of plastic off. With bags it is easier to use more than 6 inches so no facer would be required but with a bunker some care needs to be done to make sure you don't introduce air into the pile before you feed it so probably a face would be best. lots of ways of doing it, you'll have to decide what you want to invest to get it done.