Steps to have your own personal relay as a double outbox/inbox relay:

Go to https://relay.tools

Click Start a Relay

Select the standard plan

Log in with a NIP-07 extension

Pick your subdomain name for it

Hit Deploy.

After paying, the screen will redirect you to the relay settings.

Go to Access Control Mode and choose the current setting to "Block By Default and Then allow What I want".

Then on Access Control Lists / Allow Tags

Activate: Allow Events Tagged to PubKeys

Then on Allowed PubKeys, add your key

Now all you need to do is to add this new relay to your Public Outbox AND Public Inbox relays on Amethyst.