Seems like the OP's question/situation is answered, but I thought my responses might help others who have similar questions and are reading through this thread.
I do have a top on my 10 gallon betta tank. I do not have a tops on my 29 gallon or 45 gallon tanks. All of these tanks are cheaper standard framed aquariums. I have to top off my uncovered tanks with approximately 2 gallons of water each, each week, from evaporation. I have 4 amano shrimp in the 29 gallon and have never experienced them trying to climb out.
I've had amano shrimp climb out with a 1.5" gap of water level. They can easily grip onto the corners of tanks or climb up lily pipes if they are desperate. (This has primarily happened when they feel predated by large fish or the tank parameters are off e.g. elevated CO2, large pH fluctuations, etc.)
Plexi does not absorb water and lexan will bend almost the same thickness is the determining factor. This size tank with no weight to support the differences are not measurable.
Lexan however is easier to work with imo. Less prone to cracking when cutting.
I would strongly recommend against Lexan (and also acrylic). There are much better options out there IMO that just require a little bit more work. You can get glass cut at your Lowes/Home Depot equivalent and sand down the edges with sand paper (or even better, a Dremel). You can buy a lot of aftermarket aquarium lid clips for rimless tanks on Amazon, Ebay, etc. I personally like the ADA hooks, even though they are much more pricey than the acrylic/plastic ones you can get elsewhere, because they are much lower profile.
[QUOTE=Fallon;11197849]Evaporation was an issue for me to I made my own lid and bought some fittings which sit inside the rim so to speak which although not perfect doesn't damage the minimal look too much. It stays clean is easy too move, really happy with it.
One of the problems I've had with evaporations is that it steadily increases the TDS of your water. This is problematic for lazier aquariusts (like me), who don't like to do water changes more frequently than once every couple months. Nowadays, I just top off my tanks with RO water since the TDS on my RO water is ~20.