Can I make up my own extension cord for use on the job? Look at 240.5(B)(3).

Sure, the NEC doesn't apply to cords that you are using on your job site. It is an installation standard. [90.2(A)]

Your right Ryan. For some reason I was remembering the section that used the words "Listed cord sets" and when I did find it, it was "Listed GFCI protection". Must be the CRS kicking in again.

OSHA rules may apply.

OSHA 1926 subpart K would apply, The wire would need to be 12AWG and the connectors on the ends would need to be rated for use in that application.
The practice of making up a 2g box on the end would be unacceptable.

BKey, I believe the # 12 minimum is the smallest cord that is allowed to be repaired, I don't think it pertains to making a cord.

If it does can you post the exact section?

Roger