Depending on how high you need it to be, you could sink 4x4's into the ground and build off of those and the tree. OR, we have also used 4x4's or 2x6's coming down into the tree at a 45 degree angle off the platform's frame work. All kinds of possibilities.
We built a huge boxed in stand, 31 feet to the bottom of it, in a big hickory...windows, doors, roof, the whole nine yards. The next year a storm came through and absolutely wrecked the tree, stand and all. We spent 4 weekends in 100 degree July heat, hanging in the tree like monkeys to build that damn thing...just to have it wrecked in the end. I vowed NEVER again. Now I hunt out of store bought lock-on's, ladder stands, climbers, or ground blinds.