You can essentially accomplish a raft for supports by following these steps
1) change first layer width to 200% and first layer height to 150% or so. That will print the first layer extra wide and thick just like the raft.
2) you probably also want to lower the first layer speed to 30% or so which can help with adhesion (the raft does this too)
3) finally, I would also use 1-2 support base layers, since that will help give a wider contact area for the bottom of the supports.