staying buckled in

My 2 1/2 year old knows how to undo his carseat belt - he is in a 5 point harness still - and i cannot do it up backwards so it is against his body. He does not do it when we are moving but as soon as i pull over he pushes the button and is ready to get out of his seat - but sometimes you are just pulling over and not actually getting out so i have get out and go around and put him back in.   
Any great ideas on how to fix this - i know i have seen things that go over the buckle in the past but i can't find them on the interweb.

I've had similar problems, I've found "houdini stops" (about $15 from BabyCity) helpful- they're an extra clip that goes up higher on the 5-point harness, designed to stop them from slipping their arms out but I've found that they stopped undoing their belts early too as they have to wait for me to undo the clip anyway. They did figure out how to undo the Houdini Stop, but I guess they decided it was too much effort to undo two clips. I haven't been able to find those covers either.

Ella went through the same stage late last year. Like peanut we got a houdini strap but it took her 2 days until she figured out how to undo that as well. Basically only time stopped her doing it. Every time she got out I had to pull over and I explained to her how dangerous it was, I got her daycare teachers to talk to her about it and how the Police will stop Mummy and she will get into trouble, The main thing was keeping her distracted during the car journey (Tom's arrival helped a lot with that, as she nows talks to him in the car) and eventually she stopped doing it.