How to hide slider bar and close button under the SwiftUI panel

It's not possible to hide the move or close button. If you are in an immersive space you can use attachments and the glassBackgroundEffect modifier to achieve something similar to a window that is not movable or closable. Here's a snippet that does that:

struct SomeView: View {
    
    @PhysicalMetric(from: .meters)
    var halfAMeter = 0.5
    
    var body: some View {
        let attachmentId = "some-attachment-id"
        
        RealityView { content, attachments in
            guard let attachment = attachments.entity(for: attachmentId) else {return}
            
            // Position the entity as you would any other
            attachment.position = [0, 1, -1]
            
            content.add(attachment)
        }
        attachments: {
            Attachment(id: attachmentId) {
                VStack {
                    // Your content here
                }
                .frame(width: halfAMeter, height: halfAMeter)
                .glassBackgroundEffect()
            }
        }
    }
}