If your using compact florescent lighting then you can place the light right on top of the plexi. If the tank is getting to hot, then you just raise the light. My plexi warps slightly and occasionally I just flip it over to start the warping the other direction. If it is cut correctly to size then it shouldn't warp.
If you use too intense a bulb then you may cause plexi to melt but then use glass instead. The whole idea behind covering part of the top is to help keep the humidity in. An open top loses humidity fast and a light of any sort burns off the humidity too.
as for the plants and a tank like above, there is not real maintenance except cleaning glass and spot picking up droppings since the droppings you don't see will fertilize plants. If you use plants, then they must be rinsed off completely including the fertilized soil. You just replant into the plantation soil and use the 6500K daylight bulb for the plants.
I think your going to find your frog is going to grow out of that tank quickly. I had 4 WTFs in a 18x18x24H Zoo Med and it never seemed like enough for them since they were extremely active at night.
Here is a pic of that viv:
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