Kooper has frequently worked with composer Henny Vrienten, including in the band Doe Maar and as arranger for TV projects. In addition to his work with Dutch bands (including leading the band Sinister Project and playing with Coopertest and The Magnificent 7), he spent several years as sax player for Sister Sledge during European tours.
- Vriendjes Voor Altijd (2007) - saxophone
- Leuke Liedjes uit Sesamstraat (2008) - saxophone
- Ik Vrolijk Mij Op Met een Liedje (2014) - saxophone ("On the Market" song)