My cycling computer is so old that the power button disintegrated upon pressing. It was still working and I could connect it with the Garmin Connect app on my phone. I am still able to push the little switch inside the opening left by the button using my finger nail. However, this means it is vulnerable to water damage.
Replacement parts from online stores
My first instinct was to look for a replacement case from online stores and I found some on Ali Express and Shopee. It was going for around SGD$65. There is also a pretty good Youtube video that goes through the replacement of the back cover step by step.
Torx screws - too small
Fortunately, before buying the replacement back cover, I tried to unscrew the backcover following the steps on the video to see if I able to perform the repair works. While I was able to remove the torx screws from the cover, I discovered that the set of torx screws holding the LCD in place was smaller than the screwdrivers I had. I could not proceed further.
Idea of using an Eraser
I was toying with the idea of using an eraser as a quick fix for several days prior to opening the back cover. This was the time to exercise the alternative plan. Taking out my mechanical pencil (my favourite pencil for many years), I cut a section that is long enough to cover the opening. After which, I shaved it down with a pen knife to almost a tight fit and finally "rubbed" the eraser down to size.
The hack works
To my pleasant surprise, this hack worked very well. I was able to turn the cycling computer on and off. As it is a push fit and with the eraser button being slightly larger and soft, the opening should be pretty well sealed. Saying so, I might get a cheap silicon cover for added protection from the rain.