Hi there! I just bought a cot but only realised after that the matching mattress option isn't very comfortable. I'm looking at different options but due to the odd dimensions, am having a hard time finding something. I was wondering if it is safe to stray from the recommended cot mattress measurements and if so, how much can I stray by? Thank you so much!

What do you mean the cot mattress isn’t comfortable?
It is important any mattress that a baby sleeps on, whether cot or bassinet, is firm, clean, well-fitting & flat.
It is preferable that the cot mattress that comes with the cot (ie from the manufacturer) is used, however, there are times a new mattress needs to be bought.
The measurements for fit of a mattress are decided on by Product Safety Australia (ACCC) in order to prevent baby becoming entrapped between any spaces.
Information about the recommended size of the mattress to use with a cot is required by law in four places: in the leaflet that comes with the cot, on a swing tag or label, on external packaging, and on the cot itself.

Only use the mattress specifically designed for the cot, and never place an extra mattress in the cot.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/household-cots

This brochure may also be helpful.

the mattress must fit snugly to within 20 mm of sides and ends
https://www.productsafety.gov.au/system/files/Safety alert — Cots.pdf

From Red Nose:

Ensure the mattress complies with the size and depth recommended by the manufacturer of the cot. A baby or toddler can get stuck in gaps between a poor fitting mattress and the cot sides. This is especially dangerous if their face is trapped and covered, or their neck is restricted in any way. Make sure there is no more than a 20mm gap between the mattress and the cot sides and ends
https://rednose.org.au/article/what-is-a-safe-mattress