What bike is it for? I have 3x tank bags: the dedicated x-Country one, which is only about 12 litres, but will take D5 and 24-70f2.8, plus 70-200f2.8 and a flash gun, or 2 and remote flash trigger.
For the old 80GS I have an Oxford Lifetime Luggage, magnetic tankbag that I bought in the late 70s for the KH250, it is still perfect, no nicks or tears and the rain cover is still without any cracks and totally rainproof. I dread to think how many miles it has done, probably coming up for 1 million on all the bikes I have had in that time, I put 400,000 collectively on the 3x R1150GSs I had, and 334,000 on the R100RS.
It will take more than the X-Country one because it is layered. Selveral bags zip onto each other, so for a day trip you don't have to take all the lot. Very versatile.
And here it is on the R80GS - you can see how the 'day bag' expands up into a more voluminous tankbag, but if that is too much for what you are taking, zip it down to low profile like it is on the 1150Adv. - That is the same bottom section in both situations, 1150 zipped down, 80GS zipped out - it folds down like concertina inside the zipped bit.
Modern bike, 650 V-Strom I have a GIVI X-Stream 20 litre, which does much the same thing, zips to expand upwards. It sits on a dedicated GIVI fuel filler ring. In this guise it will take the D5 (or D750) and 24-70 plus 70-200 and a flash, plus some nibbles and notebook. Outer pockets each side.
