Though I technically am somewhat familiar with electrical engineering concepts (I'm pursuing a computer engineering degree), it's not my strong suit. In particular, trying to figure out how to design a device with both a sensitive radio antennae and a strong radio emitter operating on the same frequency range without interference issues escapes me (it may be possible with a well calibrated feedback circuit, but this is pure conjecture on my part; someone else would be able to help you better than I can).
Regardless, your capacity to build a signal booster yourself would depend significantly on your experience on dealing with circuits. Do you know what an inductor is? Can you read a circuit diagram? Do you know how to use a soldering iron? In order for anybody to help you on this, they would first need an idea of how technical they could be with you while still being understood, and they would also need an idea of the level of detail that needs to be expressed in directions. Depending on your level of experience (or lack thereof) and the complexity a DIY solution would entail, it's possible you may be better off saving to buy one instead.
