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!
Foam can be fantastic for armor bits, but not really a fan of it for helmets, really really hard to get it to shape well, and even harder to fill gaps. Bill there at Punished Props is an absolute legend for foam work, and even his helmets came out pretty... ehh looking.
For budget scratch builds I'm a big fan of pepakura, for just a custom mando your best resource is gonna be mandalorianmercs.com, they're greatly equipped with files and guides for low budget scratch builds.