MikroTik's wiki offers an almost-functional script to implement CG-NAT. The only changes needed to make it work are to add

:global sqrt

into the top of the second function, and to change

:for i from=0 to=$x do={

to

:for i from=0 to=($x + 1) do={

The script on the wiki does not create the xxx jump rule for the last /30 of addresses due to the counter mistake. With these two changes it works correctly.

Inside this repository is a fixed version of MikroTik's CG-NAT script that should work with fixed jump rules.