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[yokaiy]

This.

So I'm playing with the idea of doing a cosplay of Finn, but I really want to do one of the alternate outfits and not his normal one. My favorite episode is the one where he gets turned into a foot, so... I kinda wanna try that one. But I doubt it's even possible, haha.

Any ideas how that might be done?

[Penlowe]

look to mascot making tutorials, nearly anything can be crafted from big sheets of foam and covered with fabric.
The real question is: what scale would you make it?
how will you move around in it? would you do it so that you are the size of fin there in the picture? your legs probably aren't long enough to walk you around if your whole upper torso is sticking out the front like that.
Or, conversely, if you made it so that you wore the whole thing over you and Fin is just a doll sized piece out the front, how will you use your hands? there aren't any appendages suitable for you to put your hands in that would be to scale.