you are WAAAAAAAAAY better off with getting a R4 instead of a 300R. better cabling, better IO, better fans, and would be more sturdy than adding foam to the case. and the foam that you buy isnt as good as the fractal stuff in terms of quality
so just get a R4
and id recommnd this power supply. dont cheap on the power supply. its the component that keeps the system alive or just completely kills it.
Are there any other budget cases that you'd recommend that run really quietly? If there are no other cases cheaper than the R4 (110) I'll stick with the R4. Silence is a bigger priority than airflow; I won't be OC'ing.