The Mylar or PET used in Cuben is just a bear to glue, to itself or anything else.
In the past, there seemed to be a consensus on this forum that Loctite Hysol 2 part urethane glue is the way to go with Cuben. Fastenal sells it in the low viscosity, or LV version, which is good with fabric. As with most adhesives, clamping usually helps. I've found it is much more sensitive to the need for precise mixing of the parts than epoxies are.
For something simpler, Joanne Fabrics sells an adhesive by Beacon called Tiger Bond that is pretty good with mylar, but not as good as the Hysol.
Everything else I've tried, allows the Cuben to peel off fairly readily, including epoxies, glues supposedly for plastic, regular urethanes like Elmer's GlueMax, contact cements, and a bunch of others.
Am not familiar with Dyneema X, but don't see how it could be any more difficult to bond than Mylar. If it is urethane coated, a urethane glue should bond to it readily.