Open Disclosure: I haven't done this, but I've seen it done this way. It is a cheap DIY method, but the commercial umbrella holders are more solid.
You can use hose clamps to the frame. If you take some tubular webbing and run the metal through the tube of the webbing, it will protect the raft from abrasion. Tuck the long end of the clamp back into the tubing as it comes through to keep everything covered.
Get tubing from REI or similar location. Obviously, don't melt the edges so the tube remains open
Assuming you have two attachment points (one at the top of the frame and one lower into the boat) you have link two hose clamps like links of a chain and have one around the frame and the other around the umbrella. That helps reduce the awkward twisting of putting a clamp around two round pipes. If the top attachment is on a horizontal piece (so the clamps are at right angles) you shouldn't have too much play.
If you do it like that, you could make a PVC cup out of a larger piece of PVC and an end cap (glued together) and clamp that into the lower position to accept the bottom of the umbrella. The upper clamp can be just loose enough to slide the umbrella pole into and that way you have pretty quick take down with relatively little play in the set up and you don't run the risk of stabbing your boat with the end of the pole.
Also, here is another option: