Hi!
Got a bit of experience in this department.
Regarding the size, generally I'd recommend cable pass thru slots or holes to be at least 3/4" (I use 7/8" just to make sure I can pass 3/4" OD watercooling tubing no problem), so factor that in in the extension of your tray. Also, tho, lots of motherboards (for example ROG) are extra wide.
Regarding thickness, I personally recommend against Home Depot or Lowe's. The quality is poor, and the sizes are undersized (for example, typically their "1/8"" is actually .118"). The poor quality means poor machinability and low melting temps. 2 situations where this commonly bites you is when cutting with a dremel or jigsaw, the plastic will craze and melt around the blade. Second is drilling a hole near an edge--will shoot a crack to the edge. Also, generally, their ability to cut panels is very poor resulting in fractured, chipped, and uneven cuts.
Personally wouldn't recommend 1/8" thick acrylic for a tray (and really probably not .118") unless you add some central support. If it isn't going to have much support, go with 1/4".
Extruded acrylic is plastic that is made by pushing semi-liquid plastic thru a die like a Play-do extruder. This process makes the the plastic very brittle and difficult to work with. Cast acrylic is poured into a cast then let cool and popped out of the cast--this retains the best strength. I don't think they make extruded acrylic sheet--if you do happen to see it, run away.
Finally, if you've been to Home Depot, you've also probably seen a product known as Lexan--which is polycarbonate. Polycarbonate is a bit stronger than acrylic, but more expensive and available in fewer colors.
Navig