You don’t need to have mechanical innards to roll around like BB-8. Do what Imgur user jtoeman did and construct the droid from paper mache. He used an emoji beach ball for the body — he had to use something big enough to fit around his frame so small balloons wouldn’t do the trick. He then used paper mache techniques. It took a long time to cover a 56″ ball completely. He used large strips of paper, but even with that adjustment, the process was still time consuming. He used a yoga ball base for BB-8’s head. He had to use a space heater to help with drying time. A couple in-progress photos:
See a step-by-step of the entire build at Imgur.