If you want slow setting animal hide glue that does not need to be
heated actually, you may add thiourea to your glue when making it.
Or you can do what the old guys always did: you may make your glue
fresh by using the first morning pee.  Urine was used to make hide
glue if they would glue in a soundboard and needed the glue to not
set up when it cools.  This works.  I have used this method to glue
soundboards in and a few other applications over the last 30 years.

However, I would not bother with using this on your Duo-Art expression
box.  I sand the two sides totally flat on glass and put a good leather
gasket on the top half (a bit thicker than pouch leather) then add two
or three extra screw holes to the box and screw it together with 2-inch
screws.  You would be condemned to rot in "player piano hell" if you
used Titebond glue on a Duo-Art expression box and we wouldn't want
that.

I just rebuilt one of these three days ago and it works quite well.
I actually have three Duo-Art expression boxes on my table as we speak,
all of which have been restored using this method I have used for
several decades.  In fact, if anyone needs their Duo-Art expression box
rebuilt I can do it for them (for a fee, of course).  It takes only
a day or two once we start on it.

Doug L. Bullock
St. Louis, Missouri, area
http://thepianoworld.com/