Invest in a Dremel or similar tool.
Buy extra of the Lexan and practice. The small attachments that work with the Dremel are just right for that sort of thing. My son has made a couple of covers for several tanks with this method. He can make nice rounded corners, ease the edges, make nice cut outs for equipment... Any of the 'plastic' (acrylic and all its relatives) are pretty easy to work with on this small scale.