[olegkikin]

Other than the satisfaction from the DIY process, this is a massive waste of time (and thus, money). Instead you can get something waterproof and ultra light.

[pja]

That’s a rucksack cover, not a rucksack.

[JorgeGT]

It appears to be the correct order of magnitude for a cheap backpack: https://www.amazon.com/Mozone-Lightweight-Resistant-Backpack...

[JorgeGT]

And in any case, you seem to require pre-made straps.