For a mesh bag for swim toys, a basic speedo one I picked up for something like £7or8 at a swim event fair about 7 or 8 years ago. Still good. Big enough for fins inc a monster mono-fin for dolphin kick (and just pkaying about - hey doing this is supposed to be for fun yeah ?
Transition bag ? Well its on the big side but means I can fit all the shiiite i want in it when travelling is Orca's transition bag. Not cheap but Ive had it probably 5 years now, use it virtually each day and its still hding up fine. I'm ok with it as I'm 6ft tall. It could swamp a small person tho.
https://www.orca.com/gb-en/transition-bag/