The myth is busted because regardless of the shape of the raft, or slide...
unless you're MacGuyver, and brought along a handful of hauling straps, and were able to wander underneath the raft, to provide some sort of decent connection to itself to provide a way to hold onto the raft, you're not going to be able to hang on.
Will even a circular raft simply glide down semi-safely? Sure. so as you're plummeting to your death because you couldn't hold onto the raft, it's gently swaying in the breeze as it falls at a leisurely 25 mph, compared to your what was it.. 147 mph?
to JMC, in the scene in question, it's only two people, as the pilots have jumped out the plane, leaving only Indiana and the Girl in the plane. They haven't met Indy's sidekick yet. That's not until later when they reach China.
I do think however, two people *might* alter the situation, as you could supposedly put one person on each side, to try and balance the weight, but again... you've got nothing to hold on to, and nothing to keep you in the raft.
On a side note I wonder if the Mythbusters attempted another fall of a simpler variety.
They were so worried about testing the "Myth" style of it (with the raft turned rightside up and people inside of it.) that it didn't appear they tested it intentionally upside down. With the inflated "Bumpers" and the taut center, assuming again that you could figure out some way to hold on to it, it would act like a large cupped hand, with enough resistance to slow your descent, and as long as it didn't flip over, the bumpers would potentially cushion your fall.
In any case regardless, the odds of this ever working would rely on your ability to hang on in the first place, and even assuming a best case scenario, where you jumped out of a plane with your life raft in tow.. the *lightest*, and after some quick study worst of the bunch of rafts available the Survival Products RAF1104-105 weighs in at 27.2 pounds. However, this isn't the raft they used, as it's a small square single-tube raft. The one they used is more in line with a Hoover FR-6, with two tubers, and 22 inches of depth on the tubes for buoyancy. This lovely little toy weighs in at *60 pounds* .. now you're grabbing a 60 pound weight, and then jumping out of a plane, and trying to keep hold of it. Even if you did, the way all life rafts are designed, even the slowest point (when it was free falling without weight, it was still moving at 20+ miles an hour, and while the person did apparently survive, they broke their legs and arms, according to the shock watches. Realize it's not a padded "Comfort floor"... it's not going to absorb any energy of the impact. unless you've figured out how to hang on to the tubing side, without flipping the raft over, you're still going to hit the ground running so to speak at 20+ MPH. That's going to hurt.
So Good luck.
Regardless of the potential for it to work, the sheer number of variables you need to accomplish this feat are impractical to say the least, and clearly leaves this myth ultimately "Busted as presented."