I don't have an additional antenna on my RF receivers. I put a simple single 17cm wire antenna on the transmitters. I get 200meters reliable transmission through a few trees.
Since your goal is just 15-20m, and some people here seem to disregard your project scope and are telling you to waste a lot of space, I'd take all of the advice with a grain of salt.
The fact that you only get 2meters as-is is pretty low. You showed several devices. Which one do you have? What transmitter are you using? What messages are you sending? What are your power sources? What actually is your project?