Sit/stand desk incompatibility with non adjutable mac monitor

Every sit/stand desk I have reviewed does not have a height adjustable shelf attached under the top shelf, or offered as an attachment. The Mac monitor I have has an L shaped base that is not adjustable. So when I stand, the sit/stand desk can be adjusted to fit my height but when I sit down the keyboard shelf is the right height and the monitor sits 4 inches higher than is comfortable which leaves me craning my neck. Does anyone have suggestions for a sit/stand desk that addresses this issue, or at the least know a company that offers attachments such as a small height adjustable tray that I can attach to the back of a sit/stand desk that will allow me to raise and lower my monitor the correct height for when I am sitting?