I have the German army daypack. It's similar to your drawing minus the secondary compartment. The two side compartments may not be a perfect fit for Nalgene and SIGG type bottles. You will get them in no problem. But they may bulge into the main compartment space.Square sided nalgene bottles are available, so you may want to measure and look around. The side pockets are sewn with the top and bottoms open to carry skis and poles. This makes a handy load point for a smaller axe. My ruck has a inner sleeve with velcro straps against the inner wall. This is handy for a poncho or tarp and helps pad your backside. Aside from those points I merely fold or roll clothing very tight and use inner sacks to seperate everything. A daypack is just a bigger PSK. If you need the first aid kit it will be on the bottom unless it gets cold and then your mittens and balaclava are. Like my PSK, I deploy a bandanna/ poncho on the ground, dump and grab.I don't lose anything and save time if it's an emergency.