Well since you already have it you may as well try it. I recommend doing several things
1) If you are drilling or using a hole saw have it run in reverse so that you are not so much cutting as grinding the hole. If you don't I don't care how careful you are more then likely you will chip the edges of the cut or worse case just crack and shatter the entire thing.
2) Use coolant or a lubricant as you are cutting. For acrylic, I recommend either dipping the drill bit in a white candle to get some candle wax on the bit or for a hole saw I use water mixed with liquid dishsoap (2:1)
3) If You are cutting the Acrylic Mask the area you are cutting using Tape this will help reinforce the cut. If the acrylic is 1/8" I recommend scoring and then snapping along the line rather then trying to use a saw. Line up the scoured line over the edge of a straight edged table or work surface Be sure to clamp the piece onto the table with a piece of material (Preferably squared) just behind the line that you intend to snap. And apply equal force over the entire piece.
4) If you are using a dremel again work the tool backwards. DO NOT CLIMB CUT IT. Meaning if you are cutting a circle and the cutting bit is rotating clockwise make sure you work the tool in a counter clockwise direction.
5) After you make a circular cutout for say a fan or anything for that matter If you have a dremel sand the edges and make smooth any little inperfections and micro cracks. These can turn into a big crack if you accidently bend the plexi too much.
The above recommendations will help keep the tools from snagging. Granted it will take a little longer to finish but if you do it other then what I recommend you run the risk of breaking the piece and having to start over.
Just remember with acrylic more then with other materials let the tool do the work and do not force it and you should be OK
I hope this helps,
EC