Obviously, such a totally-fireproof passageway and bin must be provided at the time of installation of the fireplace. This option on a JUCA is just a pass-through opening (roughly 4.5 by 9 inches) in the floor castable firebrick. It has a short stub connector underneath the floor surface to connect to a heavy metal or masonry passageway which would then be fabricated on-site.
This JUCA option also includes a cast-iron flip-up door assembly which just sets loosely over the opening to cover it during normal operation of the fireplace.
The JUCA Home Page is at: http://mb-soft.com/juca/index.html
E-mail to: JUCA1@mb-soft.com