I have been summoned...
I have a few options:
1) OK you need a really STOUT an SHARP knife and a straight edge you can clamp down on, run a line along the edge fairly feintly at first just to establish a Kerf for the blade and have it clamped down to a table top / work bench with both sides of the cut until your all the way through. This is slow an dont try to force it or you'll crack the work piece.
2) Vibroknife / Ultrasonic cutter you can get these cheap and will cut plexi like nobody's business get a old CD case an practice on it first - I got mine for 25 and use it for cutting various infills or materials like plexi when I have to cut it, use the same process as before.
3) Hot knife, this is little more than a high powered soldering iron with a scaple blade in it, it's not as nice as the ultrasonic knife option and you can take a little longer but your less likely to crack it.
Now Tips -
1) Get a small bad nail put it in a bit of wood so it's sticking out a fair bit but it's trapped in and heat it till it's a dull red and slowly make holes along your cut line ever 2in or so this will stop any cracks from escalating by giving them a stop point, this works like off knocking when your knapping flints, Plexi works in the same way as flint when it comes to shock stresses just in a far less desirable way.
2) Practice, get offcuts an use old CD cases but whatever you do practice, Plexi is notoriously hard to cut well without special machinery or lots and lots of skill.
3) DONT be tempted to use a multi tool like a Dremmel or some form of other cutting tool, it will set up a vibration an crack along the way that you will not be able to recover from.
4) Get it cut for you by the supplier, you can find places that will do custom cutting for you it just costs a little more but honestly this is the way to go for 1 offs or odd shapes as long as you can give them a semi accurate diagram
5) Use ANY other material you can - I hate working with plexi I legitimately hate it to the point I'd rather slap my todger in a bun slather it with ketchup walk into a cage containing a very large very ill humoured XL Bully cock slap it an say Dinner time fido than work with the stuff.
