Funny that one of you even quoted the original message yet your suggested solutions do not satisfy the requirement: "yeah it's probably a bad shell,
I just love the shell so much because it lists all the songs and rrr."
But CDs don't have hiss and wow and flutter and cannot not be chewed, and they don't need Dolby B or Dolby C... And what the OP will do with his walkman? Throw it away? No, no, no! This is just not fun. You cannot be serious.
Buying the same album on a cassette is closer to what the OP wants, but maybe he likes to keep
exactly that shell? Here is an interesting topic: what is more important: the shell that one held in his hands for a hundred times, or the tape housed in that shell? This is not a question about media or sound quality, it is about what brings more memories? One does not think about the tape inside a shell when inserting the cassette into a walkman, but it is the tape that stores the music.
So far, measuring the openings in the bottom of the cassette and filing extra plastic seems to be the winner given the requirements.
Another thought: maybe the brass plate that holds the felt pad is damaged or cannot flex enough, so maybe tape is being squished and locked between the head and the felt pad? So, maybe also check those slits where the brass plate moves. Move away the tape and try gently pushing on the felt pad, does it flex well enough?