At the YMCA, they have the speedbag platform and top part of Everlast swivel. I am guessing i can just buy the standard everlast swivel and just use the bottom, but what size bag should I get? I am feeling more and more comfortable jumping rope, shadowboxing and beating on that wonderful heavy bag every day. I think I am ready to try the speedbag, but I might also need some general advice as well.