If you still have your Baby Trend seat, have you tried installing it with the seatbelt in the center position? I'm not sure if it will install tightly with the seatbelt in that position or not, but it it does, might that allow you to have the handle down and still have the front seats in an appropriate position?
Did you try using your knees to push the base towards the back of your vehicle when tightening the belt? That will often increase the amount of room available to the front passenger by a few inches. You'll also gain more leg room if you make sure the passenger seat isn't reclined. :thumbsup:
As for alternative seats, I have seen the Graco Snugride in there before. You'd have to plan to leave the handle up pretty much and you'd still lose some front seat space. I've also seen the Peg Perego in there. Assuming you're in Canada, I'd plan to put the handle in the rebound position. (I'm not sure if that position is approved for use in the US or not.)
Whatever seat you use, you'll want to be sure that you're pressing the base towards the seatback when tightening up the belt just to maximize the amount of room available for the front passenger. If the baby trend doesn't install tightly with the seatbelt in the center position of your car, then I know that the Graco seats will install fine with a seatbelt in that position anyways.
I have a 2004 Mazda 3i and the Combi Connection infant car seat fits nicely. Combi discontinued the Connection but you can still buy it from online retailers new and they do make a Shuttle infant car seat should fit as it is similar in size to the Connection. Also the Evenflo Embrace and Graco SnugRider are both smaller infant seats and should be able to fit nicely as well.
The EVenflo Embrace takes up considerably more room than the Baby Trend - the handle must be down in the vehicle with an additional 1.5" of clearance between the back of the handle and the front vehicle seat. The Baby Trend handle doesn't extend very far beyond the farthest point the shell reaches out so it takes up considerably less room. The Embrace is probably the biggest seat out there in terms of the room it takes up front-to-back just because of the handle position and extra clearance required.
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For the purposes of further information on this thread relevant to the OP's question in case someone else comes across this thread in the future, the My Ride and True Fit both fit great in the Mazda 3 sport. The Sedan has more interior room so you can count on them fitting fine in there as well.