A bomb proof 35 litres construction is this original German army mountain troop Cordura Nylon rucksack.
Made in Germany bei HEIM.
No zippers, no risk.
Der BW Jägerrucksack Cordura ist ideal für Wanderungen. Durch das Cordura-Material ist er äußerst abriebfest und stabil. Neben dem Hauptfach bieten…
www.asmc.de
This price means the currently still produced new product, but for a used one you have to pay round about 30 €, but slowly they are getting rare on the surplus market.
(They exist on the surplus market in two slightly different cotton versions too, heavier of course, the Cordura version is outstanding light. 850g currently, older versions even 100 g lighter. The cotton versions have round about 1,4 kg.)
That's the rucksack I currently use for my light and compact 3 seasons equipment.
It has exactly 35 litres capacity, I called HEIM and asked the boss. It doesn't have 25 litres how asmc writes wrong in the description.
It can be compressed like that, but it isn't the maximal capacity. 35 litres is correct!
In the side pouches fit (the relatively small) usual Bundeswehr aluminium bottles or civil 750 ml stainless steel bottles with flat closure like clean canteen or 360 Degrees, which I use because they are lighter. With neoprene cover they get a tight fit, but it's OK. I even managed to put the 750ml bottle in its neoprene cover in a 450 ml Robens titanium mug, but I only can get it back, because around all is a well fitting nylon bag which I pull out to get the mug. The mug fits pretty tight in the side pocket. It works but that isn't ideal.
With plastic closure the bottles are to high. You need the insulating steel/ bamboo closure. But than it fits perfectly with neoprene cover in the pouch.
The original German army folding mat, currently available only used but often in good conditions, is meant as a back pillow in this rucksack and sits there in it's own compartment.
Don't buy the bad asian copy of that mat. It doesn't fit properly in the special compartment because it's to thick!
But of course you could put clothing in this compartment instead of the folding mat.
That's nice if you use the rucksack as a pillow.