Hi expriest.... I guess you`ll have to experiment with a few different ones to find the products which `work`for you... I`m in a similar position ..have been wearing toppers and now just ordered my first full-cap......like you my sides/back are just so thin now I can`t `do`anything with it ..... the back perimeter will be sitting just below the occipital bone which is the round bone at the base of the skull which is about an inch or so above the line of the hairline.....I`ll leave that little bit of my own hair to remain in place/grow down to give a realistic `look`.....have a look through JRob`s site for definitive information on the whole area of full-caps and then you should be able to make your final decision on what path to go.
http://hairjive.com/ Paul.