I have a couple of Standard Horizon 390i units from a few years ago and the work OK with the internal antenna from both the upper and lower Helm. these are perhaps a little slower getting their first, full fix, but once they have it, no problem.
In theory, it should work fine in the fiberglass box, but the more material you keep adding around the unit, the more you will desensitize the antenna. I would have no problem putting the unit in a fiberglass box, but keep it as thin as possible.
Again, your first full fix after turn-on may take a little longer, but after that I seriously doubt you will notice any difference.
JimS on the Delta