Using what I felt was my "tried and proven" method of employing thin plastic over wood, the following photos pretty much are self-explanatory. If this method looks familiar, then you're correct. This is the same method used successfully on my previous Guillow's S.E.5 on the Vickers machine gun and radiator. I've found it both easier to use thin plastic to produce a smooth and consistently flat surface that is ready for painting, rather then going through the laborious task of sanding the wood smooth, filling in the grain, sanding the filler, checking for imperfections, then priming and sanding again. Even then with all that sanding, there's always a risk of over-sanding or sanding one end too much or not enough, the list is almost endless with potential mistakes.
The last photo shows me adding what appears to be a hatch of sorts.