How do I set who can use my Chromecast?

I had a single Chromecast in my network. So to lock it down to myself and not allow others on my network to cast to mine, I simply created two networks; one is isolated and one has isolation off. I simply coect to "my" SSID of the network.

Now we have two Chromecasts. So to allow for this new guest cast to function publicly, I had to tu wireless isolation off in the public SSID.

Problem

People can now see and cast my private Chromecast I have on a my private SSID, including the Chromecast in the public SSID. So we all see two Chromecasts.

Solution

I need people in the public SSID of my 1 single router to see only their public cast. I don't care whether the private SSID sees the public cast or not. I just need to see my private cast.

Available Equipment

• 1 single router serving 3 SSIDs (can serve up to 4) with separate wireless isolation option for each SSID.
• 2 Chromecast; one is public, one is private.

Tried

Wireless isolation: Works when there's one cast.

Firewall rules:

• Local machine firewall: Works to block viewing my cast from public Windows machines. But public iPhones and Androids can still see my private cast.

• Router firewall: only allows for port controls. Can't block access to specific LAN IP. However I tried blocking using block list of "domain names or keywords" but that didn't work.

Extra

My private cast has a fixed LAN IP. Just as all my private devices. Two SSIDs have different names and different WPA2 passwords. (That didn't thwart public people from viewing my private cast though).