Car seats that he can't unbuckle. PLEASE!

My 3 year old son has been able to unbuckle his 5 point harness for several months now. At first I turned it around so the red button was against his crouch but that only stopped him for so long.
Now he undoes it as soon as he sees our house or we pull into a car space at the shops. I have yelled, bribed, everything, every made him get out of the car once and threatened to drive off without him.
Also tried the Houdini clip, he had it off before I'd pulled out of the carpark!

SO, my question is does anyone have a car seat that has a really firm buckle? That even they sometimes struggle with pushing in?
Currently we have a maxi-cosi Goliath.

Hello,

We have the Mothers Choice victory convertible booster... I sometimes struggle to press the button hard enough to unclaimed the buckle. Would highly recommend this one. We have two, one in each car.

I bought them from Big W on sale and they still have it in stock (just checked online)

Maybe it's best if you go in and test a few. It's so frustrating when they take their belts off, you want them to be safe & secure... But they're only kids - they will test us (intentional or not) good luck smile