I've use light trusses made of plywood and 2x2's to create a space for bats or cellulose... the plywood does not need to be continuous, but more like webbing, 1 foot of ply every 2 feet of truss. If you are using a steel roof, the webs should be under the strapping. You can google larsen truss, but the first page that comes up is mostly about wall insulation.
