What is the ruling on using a drum open on one side? Is it considered forbidden like a drum closed on both sides, knowing that one of the sheikhs has permitted its use? Does his attendance at an event where the drum was used and his lack of disapproval constitute evidence of its permissibility?

All musical instruments and entertainment tools are forbidden except for the tambourine (duff). The permissible tambourine has a description that was previously explained, and what the questioner described is not among it, and it is likely one of the forbidden entertainment tools. And the story mentioned about Ibn 'Uthaymin—if it is authentic—then perhaps what the Sheikh heard was a tambourine.