Plain and simple, OP's computer won't fit the sizer and with UA's (welcome) new enforcement, particularly with something peculiar, it's highly unlikely that OP will be permitted to carry the item onboard and it's equally unlikely that UA will accept it as checked luggage at the gate.

OP needs to understand that the likelihood is that he'll either be tossing the computer or allowed a rebook the next day by a sympathetic GA.

He needs Plan B.

The maximum combined linear measurement (length + width + height) of carry-on bags must not exceed 14 inches x 9 inches x 22 inches