How is the masonite being

How is the masonite being glued, over lapping/on top of each other or butting the ends? Contact cement will hold best with a lot of surface contact.

I just use yellow wood glue.

Wood for wood glue

I have never found a better glue for wood than good old yellow wood glue with proper joints.  That means smooth surfaces and proper (but not excessive) clamping force.

If for any reason you cannot get the surface prepped properly, then I would suggest an adhesive with more body, like construction adhesive.  Contact cement probably would do no better than wood glue, in my opinion.

Gluing Masonite/Hardboard

I used wood glue to glue strips of masonite/hardboard together when building spline roadbed.  It works very well although it is kind of messy but easily cleaned up.

I'd start with yellow glue too

and if that don't work move on to more complicated or expensive schemes.