Thats funny, "fish tank boiling" ....
your clients could make themselves an antenna using a pringles can.
http://www.oreillynet.com/cs/weblog/view/wlg/448
total cost with cable is about $30 bucks. providing their wireless card supports adding an exteranl antenna. another $75 bucks for a good Orinoco Silver card.
edit: you are allowed 22db of gain per FCC rules covering the 802.11b frequencies.