A Windows Phone hardware modder named Marc, who also goes by the handle "Core Down", says he has successfully added a huge W-Fi antenna to a Lumia 435 and Lumia 520 smartphone. He went even further on the Lumia 520 and soldered in a CPU fan to the smartphone as well.
According to WindowsArea.de (translated):
"And he has opened the device only to the motherboard and searched for the operating WLAN module. After that he had to remove the built-in antenna and has the new to the appropriate contact points soldered. When Lumia 520 he has even installed a fan which prevents the excessive heating during the game."
We would imagine he doesn't have any problems with Wi-Fi reception either. Hopefully he will publish a guide on exactly how he accomplished this mod in the future. It goes without saying that making these kinds of hardware changes will void any warranty that might be available for those phones.