I don't guarantee this will work so experiment if you can.
Try borrowing a spare wireless router (or buy an old one at a flea market) and connect it to the first by ethernet.
Disable DCHP in the second router and select an IP address which does not clash with the first's. Give the second router a different SSID.
Obviously disable wireless security in the second router and choose a different wireless channel. Select an SSID which is self explanatory to casual users.
Locate the router away from windows to reduce stray signal, unless you want to host every freeloader in the neighbourhood. If you select wireless router which has a power control on the wireless you will again reduce needless coverage.