That's definitely true, I'm making an entire set of Boba style armor with sintra for 30 bucks. And I'm sure there's tons of Mercs that would be willing to do that. However they probably just used the molds they already had I guess.
I think that in reality is was done for budgetary reasons. However, canon wise, I think it'd be best to say this was some sort of off-shoot of Death Watch that just uses this style of armor, and continues to use it post ROTJ. Just my thoery.
We should just turn this into a full on research thread!! I think that the Mandalorian's shoulder's could be used for both left and right on this costume. The pouches are also the same as the one's on the Mando's (Dyn Djarin's) belt. It's worthy to note that they all seem to have different white...
If you really want to make it out of foam check out this video. Just be aware that by making it out of foam you won't be 501st or Mandolorian Mercs apporvable. If you aren't looking to be approved, go for it and have fun!