SQL Predicate to filter Accounts

Summary

I want to filter accounts during search in Sales Lead via SQL Predicate.

Content

We have 2 kinds of Account, Master/Parent Accounts and Child Accounts. We used a checkbox to determine which are the Master Accounts.

Requirement: Users should not able to search and select master accounts in Sales Lead.

The field we're using to store the accounts in Sales Lead is a standard dynamic choice list.
We decided to use the filter in DCL but upon checking on the field in Application Composer. The filter is not available and we didn't see any filter there.
So we're using SQL predicate to filter the accounts and not to show them during a search in Sales Lead.