If you are looking for a #27 plexi drill bit for #6 screws, stop looking. Use a 3/16” plexi drill bit instead. Before you do that, countersink the holes in the plexiglass, using whatever hole size you used for the original drilling (#40 or #30), so you can use your piloted countersink - then drill out to 3/16 using your 3/16 plexi drill bit. Enlarging the holes allows the plexi to “float” a bit - helps prevent cracks in the future. On all of mine I’ve squirted a bit of clear silicone in the hole before installing the screw. Provides a sort of rubber bumper between the screw and the edge of the hole in the plexiglass.
If you are looking for a #27 plexi drill bit for 1/8” pulled rivets, stop looking. Use a 5/16” plexi drill bit instead - easily found. Gives plenty of flex room for the expanding/contracting plexiglass.
In all cases, make sure you smooth all edges, including the holes you made, to the point that they look like they are polished before final assembly.