lounge seat cushions

I have just acquired a beautiful old English oak lounge suite. The timber is in great condition but the loose seat cushions have been changed to flat, boxy, ugly foam pads, presumably to 'modernise' the suite. I am lucky in one respect as the cushion covers seem to have been made for the original stuffing and are loose and ill-fitting. I think they would stand as useful templates for the sewing of new cushion covers.

I want to recreate the domed 'stuffed' look in the cushions. I guess the original cushions would have been sprung. I do not know how to do this but would be willing to try. In the meantime I have searched materials on the web and through a huge number of books and am beset and befuddled with alternatives. My choice at the moment is to use a core of 4"" Dunlop Enduro and then to glue and 'carve' other layers of foam both above and below and then to encapsulate the whole in calico before covering in the upholstery fabric. Can anyone tell me if this is the best way to go? Also, if it is, what are the most appropriate foam densities to use? Is there a better way? I would welcome any suggestions. Thank you. Jane