Sharing Custom Object over Custom Junction Object

I have the requirement to share a record of a Custom Object "Trial" to multiple Contacts in a Community if they have a connection. The connection between the two object is a junction object called "Trial Contact".

• As example, there are two Trials named "A" and "B"
  • Contact X of Account E should only see Trial "A" - 1 record in "Trial Contact"
  • Contact Y of Account F should see Trial "A" and "B" - 2 records in "Trial Contact"
  • Contact Z of Account G should only see Trail "B" - 1 record in "Trial Contact"

What is the best setup of relationships (lookup/master detail) to fullfill this requirement?

Thanks for any advice!