If it is the wood grain on the plastic that is causing the contrast concern, perhaps you could sand the wood grain on the plastic to lessen or eliminate the texture & then do your wood graining technique as it looks very realistic. When I saw your practice piece I thought you had done it on balsa wood,
Also, keep in mind that there would be some contrast in the grains as the framing wood is a different type of wood than the panels.
Also if the wood has a new coat of vanish, that would have an effect on the appearance of the grain.
You have probably taken all this into consideration, but I know sometimes I tend forget about the forest when looking at a tree. (just trying to maintain the same "timber" with that last remark )