Instead of folding the wingtip and risking a bridle tangle get a large freezer bag or two and fill with sand. Place these on the inside of the kite to hold the wing tip down without folding the tip. When you launch the kite the bags slip off the back. There are also sand bags made for kiting that are a little more slippery than a freezer bag.
Excellent point about the slipperiness of the bag being an important factor!
Another important factor is the shape of the bag. The classic sand bag sold for launching has about 4 separate compartments sewn into the bag. This type of bag can quickly become a "saddlebag" riding one of the kite lines. during a self-launch, I have had a very nerve wracking walk, all the way up to the location of the sand bag, as the trapped line slides under the sand bag, and acts to tension the trapped line, and thereby, add a great deal of challenge to the control of the kite. Upon reaching the location of the sand bag, I have had to physically lift the sand bag off the line, and throw it aside, while holding the bar against the ground, and thereby, temporarily giving up control of the kite.
Something like a beach ball, slit down one side, to allow it to be filled with sand, might make a good slippery, cheap alternative to a sand bag. It might form a good stable flat spot on the bottom, and then, maybe, even roll off the kite, when the kite is tensioned for launch.