hey there...
i manufacture tanks and sumps, etc...so i have a little experience
If you use a hole saw, use water to 'lube' the cutting...it will prevent the plastic from melting, and you'll end up with a cleaner hole. Ideally, i use step bits when ever possible...greatly reduces the risk of cracking the acrylic. As mentioned earlier...drill with different torque settings may be beneficial. But in anycase, work the hole very lightly...don't use excessive force when drilling...ideally, you want to teeth of the hole saw to graze the acrylic...take a little longer, but you won't crack it. Lastly, when the bit is nearly through...you might 'wobble' the drill...this will prevent the last bit of acrylic from scaling at the end. If you have any questions, please let me know.