According to an item named "Schematics - Mantis Scythe" in the game files, the Mantis Gauntlet was probably intended to be crafted by the player. In the final release, however, you can buy the weapon only from a strict list of vendors after a certain level (12 for Bardon, Mick, Miguel and the Great Khan armorer, 16 for Blake). Now you can craft the Mantis Gauntlet by using a workbench.

To craft a Mantis Gauntlet, you need:
3 Mantis forelegs
1 Duct Tape
1 Wonderglue
1 Medical Brace
Skill Requirement: Survival 60


If you search a bit, you will notice that a similar mod  was released 8 years ago by Yukichigai. There are a few differences from mine:
1) The Mantis Gauntlet used to have a DT bypassing effect, a minor value and a major attacks/sec speed. The Fallout New Vegas Patch 1.3.0.452 removed the DT bypassing effect, increased the value from 150 to 750 and reduced the Attacks/sec down from 2.447592 to 2.021738. Yukichigai's version was released before the patch and does not include these changes. From my little experience you need to use FNVEdit or the GECK to adjust them.
2) Yukichigai's version adds the relative schematics to craft the gauntlet. In my mod the schematics restoration is missing and the recipe is available from the start.
3) My mod has different requirements: 3 mantis forelegs instead of 2, Duct Tape and Wonderglue instead of scrap metal and a superior skill requirement (60 Survival instead of 45).

Note: you might not want to use my mod with JSawyer Ultimate Edition, as that mod allows you to craft a Mantis Gauntlet by finding a note placed somewhere in the game.