Fixing a cup for Washing
Please see the ruling of the Shulchan Aruch Harav, siman 159:10:
"For an earthenware k’li, sealing is effective only when one uses tar, sulfur, lime mixed with gypsum, tin, lead, or paste used by scribes to fasten their papers together (needless to say, glue is an effective seal), but not other substances. Sealing glass keilim is ineffective in having them considered as valid keilim, even if one uses tin, lead, or the like.
Similarly, with regard to metal keilim that are used for hot liquids, e.g., a kettle or the like, tar or other substances that will soften in hot water are not effective [as a seal] to render them as a valid k’li for the washing of hands unless one designates such a k’li to be used [solely] for the washing of hands. Otherwise, the seal will not endure. The same applies with regard to an earthenware kettle, a pot, or the like. Tar and the like are effective, however, for sealing other metal keilim [which will not be used with heat]."
We see from here that it depends on the initial substance of your Kvort as well as what you use as a seal.
The rule is that it needs to be a seal that will be able to continue to hold liquid, indefinately.
You will have to judge, based on the type of glue that you use and based upon your personal experience, as to whether the seal you put on will fulfill the aforementioned halachic requirement.