Ways to demonstrate ("show-off") contributions as an undergraduate in research
A recommendation letter is the way you get credit for this kind of work. I'm not sure why you think you're only 50% likely to get a letter out of this, it would be very unusual for a professor to be willing to work on a research project with you but to not be willing to write a letter for you.  As long as you're taking it seriously and professionally I think you should have close to a 100% chance of getting a letter out of it.