We usually mount a 50-gallon bladder to the cabin top in addition to having 2 aluminum 40 gallon tanks fitted into the life raft locker.
50 Gallons of diesel weighs about 350 pounds. That is not much weight considering the size of the tank.
The tank is tied to the rails using line with a wrap around the mizzen. We also use the supplied mounting straps and zip tie them closed as they have had a tendency to loosen.
We only do the final tightening of the straps and lines when the tank it totally full.
We use only one hose to fill and empty the bladder. We used to fill the galley garbage can and with the use of a jabsco vane puppy pump we filled the bladder. When the bladder starts to get full we “Burp” the tank by jiggling the tank and reversing the pump to get the air out. I had Amel fit a 24-volt waterproof outlet in the cockpit .
The tank we use is the commercial Nauta tank fitted with bronze fittings. The regular Nauta tank used to weep diesel.
I fitted an elbow and fuel hose to the tank which ended with a locking ball valve . When it is time to empty the tank we just add a piece of hose and dump the entire tank into the main tank.
We now fill one of the auxiliary tanks with 40 gallons of diesel and while pumping the bladder we add the additional 10 gallons.
It is very important to have the tank very full without any air , it must be tied and strapped down tight, Never empty the tank partially. When the tank is totally full it is hard and does not move one inch.
I have been doing this for many years in all sea states. We just make it a practice to empty it as soon as we need 50 gallons. We then hold it up to get the last drop out , fold it up and put it into the port cockpit locker.
We also wrap the locking ball valve with foam rubber when it is on deck and when we put it away.