Having owned and driven 3 HMMWV's, the main difference in the H1 civi truck and the HMMWV is the steering column (borrowed from Ford Motor Co) and the ROPS that was integrated into the roof structure.
If you have a typical surplused HMMWV, you have ABSOLUTELY ZERO protection from Falling Objects or RollOver. NONE
If I were to cage one, I would hire it done. I see so many muffler pipe cages, and out-of-spec welds. The cages would do nothing to help you, nothing at all.
I can almost guarantee you dont have the skill or know-how to make a certified cage. However, there are many guys out there in the hotrod/race world that can
Screw around and roll that pig with your family in there, and a $5k cage is cheap