Any BDU will work just fine tbh - whether you prefer black or camo it's up to you. And those pants with knee pads already in them can be useful too actually not only for looks but could try to secure the knee armor onto it too.
Tbh as both arms area and thighs are covered by armor could easily either saw both materials together - or even go for other alternative solutions like perhaps attaching these two with velcro so it could be removed later on and that bdu could be worn with something else when not cosplaying.
Tbh now I'm even more amazed by what WETA did. Now it explains why that ODST has some additional fabric slid over the black shirt on his elbow.
On the knee there is nothing but you can see the straps so the knee armor is more secured and won't flop around.
Sure the thigh pieces are missing the inner parts but I'm 99% that change was done so they could be more mobile for filming the trailer.
I'm even more impressed on how amazing their work on ODST was.
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PS. At least the H2A (and Fireteam Raven) variant of ODST has consistent undersuit!
Who will stop you from saying you kept your camo BDU but changed the armor? No one will dare haha.
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