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Your Matrix has sufficient room to fit just about anything... some good options are the Graco MyRide 65, Evenflo Triumph, First Years True Fit, Britax Marathon or Boulevard, Sunshine Kids/Diono Radian. Kids should remain rear facing for at least 2 years, even better to the weight or height restrictions of their seat. Any of these choices will keep your precious little one in the safest rear facing position until then.
Do you have any particular concerns? Size wise is your child taller or heavier? Do you need a seat that is compact front to back to leave room for tall drivers or passengers? Do you expect to need multiple car seats or other adult passengers in the back seat in the future?
Hope that helps! Let us know if you have any specific needs and we can help you narrow down a bit.