Given the simplicity of the shape, I'm wondering how hard it would be to make something like this
That depends way more on the skill (or lack) of the person wielding the scissors.

But a suitable mock-up "fabric" would be one of those 1/2" thick blue foamy "
sleeping pads" for camping. Cost should be under $10, with enough material in one pad for a couple of attempts.
Remember: holt-melt glue is your Friend for projects like this.

And/or duct tape.
Some spray paint afterwards would give the correct dark blue hue.
Cheers