Have a look at the multi selector buttons or any other controls that may have 2 clips on the same shaft and that you can live without.
I had a similar problem on my Aquatica D3 housing while travelling on a live aboard with no spares available. I could not dive with a button with no retaining clip (the clip failed and fell off - luckily I picked up the leak in the rinse tank before diving and avoided a major flood). I used a clip from the "down" button which held on a little arm which pushed on the control on the camera. The button itself had another clip which kept it in place and the housing water tight. The only loss of functionality was that I lost the use of that button and so could only scroll in one direction in the menue systems. I kept the arm in a safe place and replaced the clip when I got home. I now carry spare clips.
Have not seen the Aquatica D90 housing, but it may have another internal clip that you can use in that way.