To reduce reflections through a display case, get as close as the rules allow and avoid any light source that can reflect straight back into the lens. Don’t use on-camera flash. If possible, position yourself so the main light is off to the side rather than behind you or near the camera.

A circular polarizing filter can help cut reflections on glass or plastic. It won’t solve every situation, but it often improves the result.

If extra lighting is allowed, off-camera/remote flash or strobes can work well: light the subject from an angle while shooting from a different angle so the reflection doesn’t bounce back into the camera.

Also watch for practical issues: fingerprints and smudges on the case can ruin contrast, so a cleaner viewing area helps if touching the case is permitted. If contact is allowed, a rubber lens hood can help block reflections by sealing stray light, but since you said you can’t press the lens onto the case, rely mainly on proximity, angle, and polarization.