From my perspective few options are possible now:
1) New version of Jelastic has an ability to use isolated networks per each environment, but this version is not in production yet. You can wait until this version will be available in production, but I don't think that option can be good for you as the biggest waste in our life is a waste of time.
2) Write a simple JPS addon that automatically apply custom firewall rule set per each container in your environment. Such an addon can be written once and then can be applied to all your environments in future. Actually CloudScripting way helps to make automation of any level (including infrastructure behaviour / events subscription / deployments and etc.) In that way any topology modifications can be aumaticaly reflected in firewall rules and applied.
3) Manual firewall configuration using this article
Manual Firewall configuration in Jelastic
Probably can be the fastest solution, but it depends. If you have let's say 5 containers - that is fine as a temporary solution until more advanced feature will be available. If you have 100 containers - it's easier to write an addon. There are many examples available on Github JPS