You don't need to cut a taper, just a straight sided hole that is narrow enough to catch the top lip, and wide enough to allow the 18mm male part of the fitting to fully insert! Just measure the top of the joint and find a bit that's appropriate
You could also put an O'ring or bit of silicone tube on the lower part of the glass joint that sits in the hole if You can access the thinest part after its fitted to snug it in.
or if you like more complicated.....
DIY Solution, use an 18mm male female adaptor, wrap some abrasive silicone carbide paper around the male part. You can use super glue if you like but as the paper heats up with friction the glue can give.
Use Your 18mm joint covered in abrasive paper to ream out the top part of hole you drilled out so it accepts the 18mm to 14.5mm adaptor a bit more snugg.
Or buy a reamer in the right size.