im making a custom top plate for my waterblock, as i dont feel like looking for/paying $20 for a factory made one. so im almost done, just need to drill holes for the inlet/outlet. so if im gonna be using standard size barbs that are always used on waterblocks, (i dont know what size that is) what size drill bit and tap would i need? and also, the material is plexiglass, if anyone needs to know, not copper.

1/4-18 NPT tap and 7/16 (0.438)drill, better suited for 3/8 tubing and under, but 1/2 barbs are available with these threads.
3/8-18 NPT tap and 9/16 (0.562)drill ,3/8 and over barbs

NPT taps are a taper thread.
What that means is the further into the workpiece the tap travels, the larger the internal threading is.

So ,be carefull not to use the entire leangth of the tap just go 2/3 of the way in, then check the fit of the hose barbs,they should get very snug approximately 2/3 to 3/4 of the way in.

Tapping.
Every three full turns in ,back out at least one turn to clear the chips.
I tap polycarbonate dry,but you can use any light oil like WD40 for metals.
You can use Very soapy water on plastics as desired ,but it makes a mess and tends to load up the flutes of the tap

The barbs pictured are finger tight