I'm thinking of replacing some of my cover panels with this, I believe you can cut with diagonal cutters.My main tank (29g) is topless. I don't have fish known to be "jumpers" and there's a layer of duckweed that basically covers the entire surface.
Even when the duckweed hadn't "taken over" though, it didn't seem to make a difference.
Yes, the risk of contamination exists, but there aren't any risk factors that realistically would affect me (no smoking, no spraying of any kind, no predators or pests that could "get in" just because of the open top).
Only reason I took off the top is because I changed lights and having the new LED setup just laying on top of the old glass top wasn't practical, every time I opened it to feed the fish I'd have to ensure the light wasn't going to slide into the tank.
Plus, I had decided to add bamboo shoots to the tank, and they poke out and there wasn't enough space on the regular openings (for cables or the HOB filter) to accommodate that, so i took the plunge, it's been about 4 months maybe? Tank is overall nearly 8 months.
My secondary tank (10g) still has its hood on, and I may or may not turn it topless too when I change the lights on it, but only because the dwarf gourami that lives in it is an idiot and with him the risk of actually jumping out one day is more real, because as i said he's an idiot who'll spaz out for no reason at all. But he's our idiot, so I have to take care of him.