Think EVA, The Boss, and the Tuxedo shouldn't have bulky assault rifles? So did I.
So, using the FCNP file from Quiet Player Pack (Sniper Wolf) by noShigu, I made the DLC costumes EVA, Boss, and the Tuxedo have no rifles on the hips.

BUGS:
Unfortunately, the back weapons have a bit of a gap, as the Boss and EVA outfits are a bit tighter. The tuxedo looks fine, though.

The FPK files are in MGS_TPP/master/0/00.dat, unpack it with MGSV_QAR_Tool, and replace these files, either all, or the ones you see fit to:
Tuxedo: a391321f9b70.fpk(Snake) / 14c0b6baf6f7b.fpk (Diamond Dog Soldier)
Eva Jumpsuit: 2ac17a2301a7b.fpk(Zipped Up) / 20db082934940.fpk(Unzipped)
Boss Sneaking: 3538dd2f4ffd9.fpk(Zipped Up) / d8c75b2c66d4.fpk (Unzipped)
Then repack the 00.dat file by dragging the 00.inf file onto the MGSV_QAR_Tool.


Feel free to do whatever with this mod.
Special thanks to the folks at XeNTaX for figuring out which FPK was which.
MGS V Thread: http://forum.xentax.com/viewtopic.php?f=10&t=12407&start=885

Credits:
noShigu for the FCNP file
XeNTaX forums for helping me figure out which FPKs to mod
CrazyIvan12 for leg work.