Fitting rear 3 point seat belts in cabby.

Hi. I need to fit 3 point rear seatbelts in the rear of the cabby, any ideas on is the cheapest and easiest way? I know I can buy a kit from crazy quiffs and will do if this is the only option. But it is expensive so looking for a cheaper alternative really. Any help appreciated.
Thanks

I've had a little look into this but only ever seen the expensive kit from crazy quiffs  so I'll be interested if you find a solution.

Why are you looking for a 3 point belt, child seat?

I found a child seat, forward facing and for a child up to 4 years to fit my son in, now he's 3 I'm looking for another way to fix him in the car as I've only got the lap belt.

Yeah, the two eldest don't really need boosters now, but I wouldn't be happy with them in the back with just a lap belt to be honest.

I think I'm just going to sell my children when they get to big as I'm not selling the car!