The sliding door is quite a large area to cover.Its bigger than one 8 x 4 sheet and quite a fiddle to fix.Especially if you've already built a kitchen unit, which covers most of it! Like me.
So, I've used two sheets of 3mm birch ply.The bottom panel ,which was black plastic was used as a template.Before fixing ,the void was filled with a sheet of space blanket that was left over from a job.This should give a bit of extra insulation.Then using an angled drill to fix some self drilling/tapping screws.
The top section was a little harder,as there was the window to cut out and a reveal to form.The top part of the door is insulated with some 25mm Kingspan,held in with expanding foam.the board is then fixed in the same way and edged round the reveal.
Both panels have been oiled and will need sanding and another couple of coats at a later date.