This question has been asked dozens of times before and the answer is normally not well received.
While yes, you "could" make one, you probably can't/shouldn't. It's not a trivial thing to do. It's very complex, would be expensive and would take a long time.
You're far better off buying a commercial WiFi router and using it as a repeater. Something you can throw OpenWRT or DD-Wrt on to would be a plus. Go ahead and make a custom case for it, that's still something you could enjoy doing.