I hear you loud and clear my ds has been doing the same thing for about 2 weeks now and he is also only 7kgs. When we put him in it he arches his back while you are trying to clip the straps up, it makes it so hard and distresses everyone in the car. I do fill his lap with toys but sometimes it doesn't make any difference. I haven't seen those mirrors anywhere but i'll be looking out for them now and anything else they may work. Sorry that i haven't any suggestions for you but atleast you know you are not alone.
I thought i'd just add that i was on the site i mentioned and they also have a mirror so you can see baby by looking in the rearvision mirror while driving, bother for rear facing and front facing car seats, but it is plain so it wouldn't occupy baby just be for your peace of mind.
[Edited on 26/05/2007]
Sorry just realised i put the site on a different post, anyway it is www.babyshop.com.au
[Edited on 27/05/2007]
shyone, ds1 09/02/07, ds2 13/02/09 nsw