My daughter has recently learnt to roll from back to belly and I just put her down for a nap then she woke and rolled and got stuck between to cot slats- I got her out within 30secs- scared the crap out of me- is it safe to put mesh on either the inside or outside of the cot to prevent this from happening again?

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

It is important that you have moved her to her cot. Are you still wrapping her or are you using a sleeping bag with arms out?  Baby needs arms free & the space of a cot to roll well.
Always continue to place baby on back for sleep.

Some babies do get limbs stuck between cot slats when they are first rolling during sleep.
The risk for injury is very low.
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There was a warning previously of risk for entrapment when these mesh wraps were not fitted accurately.
If using this product, ensure you attach as per Manufacturer’s instructions.

**Do not use any form of bumper or mesh that is padded.

Have a read of the Red Nose information (link below) now that he is rolling . 

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)
        *By this stage it is not necessary to wake during the night to turn baby over to the back position
          Do not use any devices designed to keep baby in a particular sleep position.
  https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over