Do not buy another consumer-grade extender for that link, it's the wrong tool for a fixed 120ft point-to-point hop. What you actually want is a proper wireless bridge pair, Ubiquiti's airMAX line (NanoStation or PowerBeam) or MikroTik's wAP series will both wildly outperform any off-the-shelf extender at that range, especially through wood with no metal in the way.
Point one unit out from the house, one at the cottage, run them in bridge/station mode instead of repeater mode, and you'll get close to wired speeds since it's a dedicated directional link instead of a shared radio also serving normal WiFi clients. Should comfortably handle the camera at full resolution.
Costs more than a $40 extender, but a used NanoStation pair runs cheaper than trenching, and attic mounting like you're already planning is exactly the install these are made for.
This is exactly the kind of project that's easy to take for granted until the day you need a specific BIOS and every download you find is dead or unverifiable. Checking against emulator source rather than trusting whatever's floating around is the right call, that's usually where these collections go wrong.
Bookmarked for when my Batocera box inevitably needs a re-flash. Do you have a cadence for pulling in new releases as RetroArch etc. cut new versions, or is this more of an as-needed refresh?