I ended up buying a Mountain Designs Flow Plug 6 from my local Anaconda store (they only had the 1 model available). I also visited Kathmandu, which had lots of different models on their web site, but they also only had the 1 model in the store I visited. But, the stores allowed me to try the bags out for size. I took 4 of my fins in to the stores to test for fitment, and raised a few eye brows in the process (
"What is that man doing ?")

It can fit my 25 Tribal weedspeed, a Tectonics 31 Matrix pointer and my MUF 18 Delta. My Tribal 29 Weedspeed is too big unfortunately. Could probably have it sticking out, but it may fall out ?. The bag material is a very thin (tent ?) material. It would be good if it was more ruggedised. This will do for now until something better comes along. Thanks all, and to my team mates for the tips
Regarding bladder, I will remove the tube and block it off at the base with a stopper and use the bladder for ballast. The water inside it can get as black as it likes.