Can you not get 1L bottles in the cages behind saddle and on the frame and ditch the under tri bar ones?
Those ones behind saddle are 750ml so swap them for 1L and that's ~500ml right away.
I meant I could do 4.7 litres and when I get 2 more 1 litre bottles I will do. Unfortunately my frame is minute. I can't event fit a 500ml bottle on the down tube. I can get a Lezyne 700ml side entry bottle on the seat tube just about.
4.7 litres. A platypus would be easier but it's too late now for that