nappy for long journey?

[MrMacadoo]

my dd has been dry for just over a month but won't poo on the potty.

We are going on a long car journey so I want to put her in a nappy just in case. my dm says to put her in knickers and she will be OK.

I don't want her to have an accident and get upset, nor do I want her to regress as she is doing so well.

what would/have you done and how did it go?

thanks!

Would training pants work better? Similar to a nappy but like a pull up for toilet training.

But a washable doormat with plastic back and line the car seat with it. They are only a fiver and any accidents easily cleared up

Or a bath mat