I lay them across the front (or is it back?) of the pack. The top of the shoe should not expent above the top of your pack by more than 2" to avoid hitting branches.
I then hold them in place with two nylon staps one at the top of the pack and the other at the bottom; both with a snap buckles. Bungies will work too if they are the correct size and there is a place close to the side of the pack that touches your back to attach them. The bungi has to attach close to your back or the shoes will flop around. Of course straps are lighter. All this works fine for the normal 2800 to 3500 cu in witer day pack. If you are going out with a little daypack then, well, you need a bigger pack or carry them in you hands.
If you have a pack with compression straps, imagine unthreading them from the exising ladder buckles. Now attach one half of a snap buckle to one side. On the other side, you could attach the other half of the buckle except the strap is too short. So you can add a 1 ft extension to this strap so it is long enough to go around the shoes to meet the first buckle.
Or you can have your custom pack made with longer straps to do all this from the start.
And when your having the straps lengthened on the custom pack, asked them to make'em replaceable so they can be changed if the snowshoe claws damage them
BTW, These same straps are long enough to allow you to piggyback a pack or another pair of snowshoes so you can actually carry an injured person's pack for them without discomfort or a spent hikers snowshoes (two pair of shoes) comfortably on boiler plate or roadwalk at the end of the day.
Haven't tried to carry a person this way, but wouldn't rule it out in a jam if they were light. Did I mention this is a small Mchale pack with a full frame.