Understanding refitting a guage in a Volvo console

[Ipapadi]

Having just returned to our Trader 41 and whilst cleaning the lenses/glass on the control panels, two of them decided to fall inwards, probably due to heat degradation over the years. I probably need to find new 'blank' consoles to replace both 'upstairs'. Even if I find them, I have never understood how they attach with the 3 sided/U-shape bracket. Can anybody explain the principle and how it's done please. In defense of my stupidity in "not knowing how to", I've been sailing for over 30 years (inc across Med & Atlantic) but have never had to confront this problem until now!
P. S. The panel is the one with 3 x guages across the top, with rpm, sound alert and key switch below and indication lights in middle.