Booster seats suitable for lap belts only

[funinthesun]

Does anyone know of any make of booster seat that is suitable for use with lap belts only? I have been told they are available but uncommon.

Also, is it possible to convert cars specs to Isofix and is it very expensive? Or do i need to buy another car

TIA

I would say most are usable with the lap belt only I think it is just that they recommend a lap and diagonal. The belt itself doesn't hold the seat in it still goes around the child the same way.