I am not sure this can be done in less than a week, unless you have access to the appropriate tools. But hopefully someone you know will be willing to help.
If the raft has seats, can you find a clamp assembly (or make one) that works like this style of lamp?
I had one ages ago - the screw clamp attached to a desk or table, and the lamp was a separate assembly with a peg on the bottom that rested in a hole on the clamp. It might fall over if you get some serious wave action going but if you make it (or find one) that is adjustable you can set it up again. You may have to make a little shim for the pole if the fit is not perfect, or you could let the pole slide all the way through and then ultimately rest on the floor of the raft, but if you do that i would find a secure way to prevent a puncture, like resting the pole inside of a bucket or padding it very well with foam rubber and duct tape.
If the raft is the kind my parents have and is seatless, i would make a platform that you all would sit on. I just worry about the safety of that. Regardless, here is a description of how i would do it.
THIS IS VERY LONG. It is a text file i saved as a jpeg in order to post it here.
This is also predicated on you already owning the raft, and also that you will in fact be floating, and not white water rafting. And of course safety issues are paramount here. If anyone objects at all to the safety of having a large pole and platform in your craft, abandon project!
My only issue is that if you have to get rid of the thing in a hurry, you can't because you are all sitting on it. So use your best judgement about the type of river you are on, the ages of people in your craft, your ablity as swimmers, and compliance with life jacket wearing. (Might i stress the life jackets? )
Good luck. Please let us know how it turns out.