[Avodah] May Kohanim visit the Rebbe's Ohel by means of a "box"?

[Moshe Y. Gluck]

> R' Richard Wolpoe wrote:
> > Rav / Herschel Schachter states:  /
> >  [...]
> 
> > And now by extension may kohanim use an innovation such as a box to
> > visit the Rebb's Ohel?
R' ZS: 
> Why on earth not?   What issur are they doing?

Would someone kindly explain the Metzius of the situation? The Gemara talks
about Ohel Zaruk, and boxes, so I assume that there there must be something
else here that makes R' RW raise this question.

KT,
MYG