Is there anything on the market to extend a 5 point harness?

[Jojay]

Ds1 (3) is in a group 1 carseat. He fits into it well. eyes are below the top of the seat etc, but the harness is becoming really tight to do up, especially if he has a coat on.

I don't want to take him out of this seat too soon.

Is there anything out there that can solve my harness problem?

It is best not to keep the coat on whilst in the car seat. A fleece coat would be less bulky or just a blanket over him until the car warms up.