When I'm home, I use one with a beanbag bottom like this all the time:
http://snipurl.com/2umc7 [www_mileskimball_com]. This model is wide enough for using a mouse.

Sometimes you can find them locally in office supply stores and Bed, Bath, & Beyond, especially in August and September, or on ebay.

On the road I keep a Targus thin, folding laptop desk in my computer case. It, too, is wide enough for a mouse; but I have to use an open file folder under it if I'm wearing shorts. Otherwise, the bottom of the desk leaves funny lines in the legs.