my bong was 40$ canadian, its 20" 14.5mm gong 3.5mm thick glass the slide it came with fits very well although the female joint DOES wobble a bit(doesnt affect the tightness of the seal though(and i thought it would). I also once had an 8" GonG bong i got for 20$ in a case, im sure you're aware of the kind i am talking about, those cheapies imported from china, it also has a 14.5mm slide and its slide fits mine perfectly. In fact, i once found out, that a beaker at a surplus store that was selling for 4$ had a GonG seal top, and that plug was 14.5 mm to.

Thats not to say it cant happen,i just havnt run into it, but your cases are the first ive ever heard of with this problem that the slide is a LITTLE too big for female. Keep in mind though, id tend to think that the BOTTOM of a 18.8mm slide wouldn't be much bigger than the TOP of a 14.5mm female, could this explain the phenomena possibly?

Anyway, Id email EDIT about it, they're nice guys and will prolly help you out, send you out a proper one or something.