Kagehina:

Kage's did it, he would probably go somewhere 'formal' when I mean 'formal' they went to a park or something 

Bokuaka:

Akaashiiiiiiiii, yacht/boat idk I just seem like Akaashi would do something like that

Iwaoi:

Lol I like to think Iwa was mad of all the fangirls trying to hit on Oikawa so he made it official, did it right in front of a bunch of fangirls (still didn't stop them from hitting on him tho)

Kuroken:

Kuroo did, he got him a crap ton of games (because he's rich) and stood in front of it and asked him to marry him

Daisuga:

Dadchi did it, Disneyland with a good view of the fireworks 

Tsukiyama:

Tsuki did, I have a headcanon where Tsuki would put the ring on Yams when he's asleep and when Yams wakes up all he sees's is a ring on his finger and a ring box right next to his head.