Some X-T1 suffered a similar problem and many lost their stock eyecup early on before Fuji improved the fitting and before they introduced the long Eyecup.
I never lost the stock eyecup of my X-T1 (or felt it was too loose) but I did buy the long eyecup, now I have both ( the short is there but I doubt that I will ever need it).
I doubt that you could modify the stock eyecup of the X-T2 , short of glueing it ( bad idea!) or adding some rubber or plastic to physically make it impossible to slide it off.
Anyway, the good news is that, should you ever lose it, the long eyecup of the X-T1 fits the X-T2 and it is longer and better than the stock one.
