You can transport 2 x 9kg. bottles loose in a vehicle, and I'm not saying it's safe, but 50 Ltrs. is the limit for that.
It's common for dual 9kg. bottle setups, but it's the space required to fit more than 2 bottles.
4 & 4.5 Kg. bottles are often fitted in a similar manner to the bigger bottles.
And they have to be out in the open, and not situated in a "crash area", on the bodywork.
As well as securely fitted, even if they are just sitting there.
I'm hoping someone with a Gas fitter ticket can give a better answer.