This is going to be tricky, because you don't own the easement or property between those houses (otherwise, you might have buried an Ethernet cable to solve the problem), you likely don't have line of sight to their router, and the distance is probably pushing 400 feet or more. I assume you'd want to change the MAC address of the network adapter to avoid having the Xfinity WiFi session time out.
What you probably need is a directional antenna connected to the Wi-Fi adapter of your choice. Alfa is one company that makes both products. On eBay, you can find a number of panel and Yagi antennas with various amounts of forward gain, 20 dBi being a good starting point given the likelihood that the router's signal will be deflected and partially absorbed by glass, water vapor, and other obstructions. If you can mount a Yagi antenna in a tree and point it at your neighbor's window, do it. You'd ideally buy LMR-600 cable with the proper connectors to bury and backhaul from the antenna to the demarcation point at your home, but I'm assuming that limited money is the reason for freeloading on a Wi-Fi hotspot in the first place, so it's for you to decide which option is affordable.
As for the Wi-Fi adapter itself, you might consider buying a used Wi-Fi router with detachable RPSMA antenna ports for this purpose, since router firmware is more likely to maintain the spoofed MAC address persistently than a USB network adapter. You'd set up this router to connect to Xfinity WiFi in bridge mode. It's hard to say which link budget in transmit/receive power is necessary without knowing the distances and current receiver sensitivity for the rented Comcast modem/router.
Leaving aside the cost of cabling for now, the directional antenna and used router wouldn't cost much more than $30 on eBay, and you could borrow a drill with a $10 spade bit to bring the cable into your house from outside. Otherwise, you could first try mounting the antenna indoors or on a window (if your HOA permits it) and see if the connection is satisfactory.