So, after an incredible journey of acquiring my Lexan sheets ("5-7 working days" - HA!), I now need to start drilling holes to put posts in.
I started with my 3mm posts and drilled 2.5mm bores for them - very satisfied with the result! Easy to force the brass screw in, creating a thread of sorts.
It doesn't crack or bulge even 1mm from the edge!

However, I need to fit m2.5 posts in.
So, what bore diameter should I drill to achieve a nice thread but not "pushing out" too much material?

(PS: I can't drill and tap because most of my posts go into the sheet sideways, not through the sheet, meaning that I am not drilling and tapping the material all the way - or maybe I should drill a deeper bore and tap that, so my effective bore has a quality thread? hmmm...)

 

Depends on the potential load on the screw, why not do a couple of tests to determine the largest drill size you can get a way with?
Max.

 

"Hole size charts" are common. Here is a link to one for metric: https://www.amesweb.info/Screws/Metric-Clearance-Hole-Chart.aspx

Since you are using the screw as a tap, you may want to use the hole size for a loose fit or thread forming tap. (NB: There are different ways to thread a hole. Two common methods are to use: 1) Thread-forming taps which remove very little material but require more force; and 2) Thread-cutting taps. Using the screw as a tap is closer to thread forming. If you cut a small slot perpendicular to the threads it will be more like a thread cutting tap.)

Also, a drill is often labeled with the size hole it will make. The actual drill diameter will be a little less. That is, the hole you get will be a little larger than the drill diameter, if you measure it accurately.

For your project with acrylic plastic such nuances probably don't matter.

 

#43 should give you a tight fit, #42 or 3/32 should give you a looser fit. #41 for minimal material displacement and loosest fit.

 

Unfortunately, I don't have an assortment of bits to play with. I am going to buy it via eBay but I am also quite broke. So, I don't want to buy 3 extra bits I don't need.
2.1/2.2/2.3/2.4?
I do have a 2.0 which is too tight, doesn't screw in at all...