This would seem to be an acrylic tank upside down on a glass tank? If you made your own you could make it to sit on the lip inside the top frame of the glass tank. And, with no water pressing out on it, you could use something like 1/4 inch thick sheets to make it, so warping would be lessened.
Why should it be nearly airtight? If it were somewhat ventilated the condensation problems would be far fewer. And, if the light were a foot above the top of the plastic it shouldn't heat it enough to cause warping. (But, it would take a lot of light to get reasonable light at the bottom of the glass tank.) This should be a very entertaining project, so I look forward to seeing how it goes.
About the riparium style planters: to get a land area you could make an acrylic shallow box, supported by suction cups, or much better, by magnets, that would be as large as you want the "land" to be. Just to brainstorm: you could buy, on Ebay, a acrylic dome, maybe 10 inches in diameter, then cut it in half, to have a quarter of a sphere. Glue a flat acrylic sheet to the back and it holds water or dirt. You can buy magnets that could hold up a ton, so that would only be a problem in that handling really strong magnets is difficult and extremely hazardous.