sorry for the multi-posts but i just realized that I never actually made a real response to the OP
I just did this yesterday so I can tell you it will work.. here is what I did ( a HUGE thanks to Mike for his suggestion and the tips from everyone else

)
got the measurements of my lid
went to the store and got the appropriate size sheet cut to my dimensions..
came home and very VERY carefully drilled holes in the plexi (I went with about 5 so there was ventilation)
set it on top of my tank lid to make sure it would get my humidity up to where I wanted
the holes should be no larger than a pencil eraser
at that point I discovered my hydrometer was shot and that my humidity had been fine however the plexi does raise my tanks humidity from around 70% to almost 100%, my acurite told me HI% so I dont have the best measurment lol. But I am willing to put a lot of money on the fact it will raise humidity well for you too.
what substrate are you going to use, coconut husk and cyprus mulch are the best for humidity according to what I have found on here, and been told. I actually switched back to aspen because the plexi did such a good job.
as for your light assuming you have got to have it for your temps I recommend hanging it over the plexi at least 4 inches up..it will melt the plexi if it is too close.. trust me on that one

it would be better if you could go with out it. the other option is to cover only 60% of the lid with plexi and keep the light on the end that is uncovered.
Hope that helps you out a bit.