Starting to think about booster training. Which one to choose? (Canada)

[Boot]

My son is almost 5 and in a Graco Nautilus. He is on the top slot but will still fit harnessed for a while. He's almost 46" and around 45lb. I plan to keep him harnessed for another year-ish and then use the nautilus as a booster but I'm thinking about starting booster training with him in the next few months. My thought is that once he's in kindergarten he'll probably be asked for playdates and I don't want to lumber the parent with the huge heavy nautilus. Plus, I don't really trust other people to install it properly. I think a lightweight hbb would actually be safer, as long as my son knows how to sit in it. Plus, I sometimes have other kids in my car and I'd like to have a seat for them. 

Can anyone recommend a cheap, lightweight, easy to use hbb that fits well into the 3rd row of a 2007 Mazda5? Must be available in Canada. 

[TiredX2]

Wanted to bump this up just in case the poster is still looking.

Any suggestions?  (We were happy with the Graco Turbobooster for Car Pools).

[DahliaRW]

I'd go with a turbobooster.  It's economical and fits most kids very well.

[Boot]

Thanks. We did get a turbobooster and got it price matched for $44!! (Not sure what it is in the States but its normally about $75 here). It fits my son well and he sits still and is very proud. It think it felt weird and tippy at first though. He's still in the nautilus full time but it's good to have the turbo 'in the wings'.