put the plastic UNDER the plywood. plywood is a water sponge. 3 mil and thicker can be quite slippery when dry so BE CAREFUL of slips, the ply can skate on it also.
plastic or any material impervious to water migration on top of the bike will hinder evaporation,
(gore-tex seems to be advertised for this but I wouldnt use it if any other measures were available.
waterformed corrosion is usually the product of LONGER exposure to water on the surface so fastest evap is better.
anticondensate heaters in elec and mechanical outdoor gear are very common to defeat condensate, it usually happens when the air near the cold metal is trapped and the outer surrounding air warms (and gains relative water content) and then it incidentally contacts the cooler metal. bingo- the 'dew point' of the higher humidity air is reached and the extra water comes out on the cold metal.,
any conventional outdoor storage area is hard to keep free of waterformed corrosion so if thats the only $ compatible way - think protective chemistry, and it may not be pretty ( cosmoline history?)
you could do the zipper-balloon-bag thing. get a big zipperbag ( they do make them really big) and put it on the ply, push the clean/dry bike onto it, zip it up, ( can fill with DRY nitrogen or DRY air,) that keeps transient high-humidity (high water vapor content in the air) away from metals because there is very limited condensate potential if the INTERIOR air is dry.
zip it closed on the coldest day you can ( in times of no rain recent, its often the dryest air).
probly too extreme for budget but science can find 'catskinning' techniques in many procedures.
google car-storage-bags and zipper-bag-vehicle-storage and you can see them. this may be much faster ( but more $) than all the work to apply and remove antirust surface protectants (lots of investment in elbow grease there) .