Flush problem size

I have an older wall-hung toilet with a concealed cistern, and the mounting frame for the flush plate is missing. The dimensions of the white cistern box where the frame needs to fit are 165mm (height) x 175mm (width).
The flush plate I want to use is the Arezzo Brushed Brass Square Button Flush Plate (Part No.: FLSBBS). However, it doesn’t fit the current cistern box as there’s no proper mounting frame for it, and the flush plate mounting frames available for Arezzo compact cisterns don’t match the dimensions of my setup.
I’m looking for a solution to either adapt an existing mounting frame, create a custom one, or find an alternative way to securely install the flush plate. Could you help with a workaround or recommend a way to custom-make a compatible mounting frame?"

https://www.victorianplumbing.co.uk/dual-flush-concealed-wc-cistern-with-wall-hung-frame-arezzo-toilet

https://www.victorianplumbing.co.uk/arezzo-brushed-brass-square-button-flush-plate

https://www.victorianplumbing.co.uk/flush-plate-mounting-frame-for-arezzo-compact-concealed-cisterns