Radiator OK, plug not OK

[C10wip]

I recently installed a new Spectra radiator, the CU161 version which fits and is slightly larger and presumably better cooling than the CU162. Also, it's cheaper at AutoZone than RockAuto, which is a little unusual.
Anyway, it comes with a bypass plug/cap hose clamped on up by the filler neck. After only a month, I noticed that this plug was already cracking at the end. Not good! So, I am replacing it with a Dorman plug which appears to be a lot thicker but I am still a little concerned. The Dorman plug is part of a set, number 02253 so you end up with 7 other odd plugs. The plug ID is 11/16". Has anyone found a sturdier plug or better way to plug this off?
If you have installed one of these radiators and used the original plug, you might want to check it.