This isn't specifically a skate backpack, but I found this one is just large enough to fit my skates (women's size 7, with heel brake) and pads. I clipped my helmet to the top.

"Amazon.com | Fuel Top Load Sport Backpack with Side Tech Compartment and Ergonomic Padded Mesh Breathable Back, Steel | Casual Daypacks" https://www.amazon.com/gp/aw/d/B078Y7GQH6/ref=ppx_yo_mob_b_inactive_ship_o0_img?ie=UTF8&psc=1