[Tristan Lall]

It looks like you're using it for the bottom of the tank, right?

If so, you might want to just use a belt sander to make the rounded corners—you'll get a cleaner finish than with a Dremel, because you'll be removing material as it heats up, as opposed to just melting it due to all the friction from using the slitting wheel on the Dremel. If you can't get it cut precisely to the size you need, get something oversized, and then score and break it, rather than trying to cut material off with the Dremel, in a long pass. Or put it on a table saw. In general, trying to cut a straight line with a Dremel is an exercise in frustration, and won't likely be pretty.

By the way, acrylic can be laser-cut...if you know of any place that can do it, you might be able to convince them to do it cheaply.