Acrylic doesn't do so well as a lid - after a bit of time, it will sag to the point where it may fall into the aquarium.
I'm not sure why, I've heard some people say the humidity warps it/makes it prone to sagging, but it could just be the effects of gravity over time.
If you have any scrap glass, it's not too difficult to cut with a bit of practice, and you don't need very thick glass for a lid. (I just cut up a couple 1/4" panes the other day, and it went pretty well after I practiced on some scrap/waste) The hard part would be cutting the front part, you might be able to get away with using a rectangle and cutting the corners off, making a sort of misshapen hexagon. I've heard that curves are possible, but I imagine they are a lot more difficult then simple straight line cuts.
You would still have to get a hinge - they sell them at plastic supply shops, and sometimes fish stores. You might also want one of those thin plastic flanges/skirts that go on the back, so that you can cut out holes/gaps for filters.
You can make your own living hinge out of silicone - put masking tape on the glass up to the edge on the face of each piece, leave a bit of a gap (I'm not sure how much, 1/2"?), put a wide piece of saran wrap or masking tape spanning the gap on the bottom, and then fill the area with silicone and make it nice and smooth, and let it cure undisturbed.
Although, at that point, buying the hinges and the glass and all, it will probably be about the same cost as just buying one of those lids in the links above...