Part of my contract that I can’t work for anyone else, driving or any other employment, other than voluntary work.
I’m on a 4 on/4 off pattern but occasionally do a 5th shift if
a) I can be arsed
b) It fits in with my home life and plans
OR
c) it’s all gone massively wrong, as it does sometimes, and I’m caught out of hours.
I at least know it’s appreciated where I am and is a favour “in the bank” plus the extra cash doesn’t hurt.
As already said - if they pay enough just do an extra day for the firm your with.
If the pay isn’t enough - explain that you want to do a bit extra elsewhere and see if you can come to an arrangement.