I assume you're applying this cycle? If so, you probably don't have enough time to forge a strong relationship with any one PT. So I think your best option is to give one of the PTs a chance to get to know you better. Ask them to have lunch/coffee with you. Say you would like to ask them some questions regarding PT school and would like some advice, etc. Talk to them about what your goals are and what you've done so far to achieve them. As twsurfsnow suggested, give them a copy of your CV. Show them your PTCAS essay. Tell them your stats and where you plan on applying. It's very hard to do all this during your actual observation hours, because the PT is understandably kind of busy.
If you still don't feel like you have that great of relationship and you need to ask for a LOR, downplay the importance of it... Don't put a lot of pressure on them or make them think that your entire application revolves around their role. Assuming they have already agreed to verify your hours, it shouldn't be that much of a step to ask for a reference. "I really enjoyed observing you/I learned a lot from you during our time together. Thanks again for verifying the hours. I have another favor to ask. I need to have one of the PTs I shadowed act as a reference on my application. If you aren't too busy, would you mind doing this? It's a simple process. You'll get an email from PTCAS.org and you will rate me on a scale for several qualities, and then you have the option of writing a brief letter." Tons of professors and PTs are asked to write LORs for people they don't really know all that well. Which is difficult to do. So make it as easy on them as possible.
