I used to do the same thing (but I used the touchpad, so didn't need a place for a mouse) and had the same problem...especially when I was 9 months pregnant, and after the baby came and needed my lap! So, we bought some of those TableMate II tables. They used to be advertized on tv a lot. they're white wth white legs and they fold u to go under your couch or chair or whatever, and they can b pulled as close to you as you need it. We bought ours at the grocery store (we have really cool grocery store). I dunno if off-site links are allowed, so here's a picture.
Anyway, if you google it, they're easy to find. I have a 15" laptop and an EHD and have plenty of room for my mouse (which I now have to use 'cause dh made my touchpad not work LOL).