Like DylanMarx said, using references is important. Try using cardstock and masking tape or something similar to create a rough pep of sorts to give you an idea of the size and proportions of your piece before building it out of foam. This also helps you plan out the angles you'll need for each bit. Sometimes you just have to wing it though, only using the cardstock to plan out the next piece of your build, but this method only goes so far before you gotta start building with foam anyway. You may end up having to redo it once or twice, but it's also pretty helpful not having to worry about having the template files and whether or not it will properly fit you, since you built it specifically to fit you. Also, don't worry too much about getting every little detail into the EVA foam, most of that stuff can be added later using craft foam, just focus on the big stuff. Might be a little rough at first, but once you get the hang of scratch building, you can practically build whatever you want without the need for foam files. Definitely worth learning, sorry if I rambled a bit but I hope this helps!