The holder is meant for handlebars that are 22 mm till 31,8 mm Ø. It is possible to create a little more room by removing the piece of cork on the inside of the two rounded corners.
For ATBs/ mountain bikes/ thicker handlebars our range includes the BasEasy system HANDLEBAR HOLDER EC 70122.
If even the 70122 doesn’t work the BasEasy-handlebar stem holder 70103 or 70123 may offer a solution. For the mounting options of the BasEasy system handlebar stem holder 70103 or 70104 we refer to above question.