Backpack recommendations - big enough to put a helmet inside.

[Ben Diesel]

I guessed the capacity before I found the eBay listing. It's not labelled and I have no way of making an accurate measurement of its capacity unless I fill it with fluid.
And I have put my own Arai RX7 full face helmet inside the rucksack I have here, and closed the zip to secure it.

Just trying to help you out, that's all.