I saw somewhere on myArmoury (I think) that someone had made one out of old car tires. If I remember correctly, he suspended them with ropes from both the floor and the ceiling, so that they wouldn't move far when struck, but had some give to them. Rubber is pretty safe for blunted swords to strike, and tires will take a lot of abuse, so this isn't a bad idea.
I haven't had the space to build one, so I'm curious to see what you come up with. I'll want to make one after I move to a new place.