Hi, bub does not like portacot mattress at all and considering getting a custom cut foam one from Clark rubber to ensure it fits properly so no gaps on sides. Is this safe for babies to sleep on and what is maximum thickness the mattress from Clark rubber can be? Thanks!

Red Nose Product Safety Australia recommends that you only use the mattress that comes with the cot (or the manufacturer)
This is mainly due to risk of entrapment if mattress does not fit properly, or may be a risk if it is not firm enough.

Folding cots are designed for for temporary use.
If you need to buy a mattress, get one from the supplier that meets the specified dimensions.
Ensure that the mattress, padding and cushioning material is sufficiently firm to prevent it from moulding to a child’s face.
Only use the mattress specifically designed for the cot
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/folding-cots