Is there a car seat I can use with just a lap belt?

[ASecretLemonadeDrinker]

I bought a car seat that said it was compatible with 3 point and lap belts, but cannot find in the instructions how I can use it with just the lap part. It is this but it doesn't fit in the middle anyway with the other two. It's to seat a 4 year old, only for a few days so a booster thing would be OK but they all seem to say 3 point only. Is there anything I can use with just a lap belt? Thanks.

i take it you already have 2 other carseats in the car?
how old are the other 2?
i have a britax eclipse in the middle with a highbacked booster on either side for my older two,
could you put one of the younger ones in the middle in an eclipse or a britax prince (a bit cheaper but doesnt recline once fitted)and then your 4yo can have a highbacked booster with a 3 point belt?

if its just a lapbelt personally i wouldnt put a child in there unless its an emergency,can't you use the front seat for them?