I need to monitor cell density in a mini-bioreactor and do not have a headplate port available. Any suggestions?

I recently came across the miniBE from Buglab which I think addresses your application. It is a non-invasive solution which attaches to the wall of your mini-bioreactor and monitors cell density. A unique feature of this product is that the sensor can be reused and the system itself is capable of being multiplexed to several vessels. Here is a link to more information.