Need lightweight seat as spare

Need some advise please, I do not drive and will need to catch a ride with a friend once a week to a location about 15min away. We have a radian xtsl in our car and though narrow is very heavy. I am looking for a very lightweight seat that I can carry down my walk up apt with my child who is 28 months and 30 lbs.
Another consideration I have to take is that she already has a grace infant seat and a recaro in the backseat of her Hyundai sante fe. So it would also need to be very narrow.
I don't really want to send a ton of money on a seat that I will only use once a week for 3 months if that and maybe it will be a spare travel seat in the future. As our radian will last a long time, I'm also not really looking for longevity. FYI my lo is ff currently in his seat.
I am considering using the Cosco high back in harness mode as it seems to fit the bill all around. Are there any safety concerns with this seat or any reasons I shouldn't use for the short weekly journey?

I would NOT use the Cosco High Back booster, it is horrible. We bought that for my daughter as a back up seat and she out grew it before she was 2. It is in general not a good seat for any child, but it sells great b/c it is so cheap

I would suggest a Maestro or I believe the SureRide is pretty narrow as well(but could be totally off on that). We had to do three across in an 01 Altima(pretty small back seat) and we used 2 Maestro's and a KeyFit30 or 1 Maestro, Scenera and Keyfit.

The Maestro is very inexpensive and will last you a LONG time, my daughter is almost 4, 42". She is on the 4th slot, but only b/c the 3rd is right at her shoulders. She's got easily another 2-3 yrs in it. Plus it is an easy install and very light.

Hello and welcome! :happy-wavehello:

Yeah, the Cosco high back is generally outgrown pretty early. The top harness slots are pretty short and the 40 lb limit for the harness means it's often outgrown before the child is ready to use a booster. And it generally makes a pretty poor fitting booster. All seats on the US market have to pass the same tests, so safety wise it's not a concern, it's just that there are other seats that will get you more baff,ng for your buck, so to speak.

The Evenflo Maestro has 18" top slots and a 50 lb harness limit, and it's fairly narrow, so that might be worth a look for you. It does become a booster but most kids outgrow the booster about the same time they outgrow the harness. Iit's about $80, but you can often find a sale.

The Evenflo Sureride 65 has 19" top slots and a 65 lb harness limit. It doesn't become a booster, but you can always buy an expensive booster when the seat is outgrown. It's around $90 but you can also usually find a sale.

Both are light weight seats with easy installs.

I actually replaced the Maestro in our tight three-across with a SureRide because it fit a little better and my daughter had more growing room. I know your little one is currently ff, but you may want to consider trying a seat rf too. Sometimes seats puzzle one direction but not the other. You can check out this thread for lots more examples of successful three-across that might give you some good ideas. http://www.car-seat.org/showthread.php?t=33226

Is there a Babies R US that you could meet her at to try the seats?

ETA: I forgot about the Harmony Defender at Wal-Mart, It's narrow, 18.5" top slots, 65 lb harness limit, and it does become a booster, though I can't recall it it makes a good booster or not. It's not a light weight seat, but it might be worth taking a look at. It's $99 at Wal-Mart.

Thanks for the replies!
This is actually turning into quite the dilemma for me.
Weight is a huge factor, juggling a seat and a toddler down 3 flights of stairs will not be fun.
I am willing to pay $$$ but only if it was truly worth it in terms of functionality. Also I'd like to use it as an air travel seat as the radian is not great for lugging through an airport!

Any opinions on the evenflo surekids 400?

Have you considered the Safety 1st Go? [ame="http://www.amazon.com/Safety-1st-Convertible-Booster-Clarksville/dp/B002UP3FB4/ref=sr_1_1?ie=UTF8&qid=1378656650&sr=8-1&keywords=Safety+1st+go"]Amazon.com: Safety 1st Go Hybrid Convertible Booster, Clarksville: Baby[/ame]

It's not as inexpensive as some of the other seats you're looking at, but it might be worth it for the portability.

You MUST have a top tether anchor to use it as a harness, and it can be impossible/near impossible to install with a seatbelt. With LATCH, it's usually very very easy, though. Will you have LATCH available in your friend's car?

It cannot be used on an airplane, so that might be an issue, depending.

FWIW, I find the Radian extremely easy to travel with as long as you have two carry straps to make it into a backpack.