A good carrying spot for long, bulky items is to put it under the backpack lid (the flap over the top of the pack) placed sideways, parallel to the shoulders. This actually has some advantages over carrying on the bottom of the pack, because the weight is over your center of gravity, rather than behind and beneath it, which would have a tendency to lean you backwards. The main disadvantage is that you have to remove the top-mounted item anytime you go into the pack's main pocket.
I can't find any good non-copyrighted images, so here's a link to a page with some examples (scroll down to the 'Floating Lids' section):
sectionhiker.com