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Hello @DAZ69
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about fixings for external cladding.
If you are using James Hardie HardieFlex™ 2400 x 1200 x 6mm Cladding 2.88m² it can be fixed in place using Buildex 9 - 10 x 32mm Zinc Alloy 3 Star Head Fibre Cement Screws if you are using a drill or Paslode 40 x 2.8mm 500g Galvanised Fibre Cement Nails if you are using a hammer.
An example of a vertical timber batten you can use is the 42 x 11mm 5.4m H3 Treated Pine FJ DAR Primed LOSP, it's treated and designed for outdoor use. There are a number of sizes available, I suggest visiting your local store so that you choose which one you want.
On the condition that you'll be placing the timber strips over the studs you can use the same type of fixings, but much longer of course depending on the depth of the timber strip you choose. But what's important is that the screw or nail must reach the timber stud so that they are secure.
Here is a link to the sheet's technical specifications and installation guide: HardieFlex sheet
Let me call on our experienced members @Nailbag, @TedBear and @Dave-1 for their recommendations.
If you need further assistance, please let us know.
Eric
Morning @DAZ69
I am trying to picture what you are describing, Is it you want to put up a sheet of hardiboard, but on the viewable surface have effectivly the relief of the studs and battens?
I agree with @EricL with the fastening of the boards to the frame, The new frame (fake? frame) that you wantto be displayed could be put together on the floor then stood up and fastened at several points to the first frame(Using either nails driven in diagonaly or long screws through the cover frame and into the one holding the hardie sheet). I am thinking like two pieces of same cut bread with the hardboard sandwich.
Just not sure If I have it right? Any chance of a photo or a sketch? 
Dave
