From your one Momomegu fic, Momoyo likes being buried because she finds it comfortable.
Tsukasa lives in a tube, which is likely tight and constricting in the same way which she would probably like if she goes in there by choice.
Ergo, I propose that the feeling of being compressed is one of the only things Momoyo and Tsukasa can agree on being good other than Megumu.
it's like when u roll a dog up into a blanket like a burrito










