Out of curiosity, I tried this with my prebuilt Mac OS 9 system (which you can find by searching for "Mac OS 9 for OS X/MacOS"). I did this:
1. I downloaded the same SIT file that you downloaded from Macintosh Garden.
2. I started up my Mac OS 9 app.
3. I dropped the SIT file on the icon for my Mac OS 9 app. This copies the SIT to the SheepShaver desktop in my system.
4. I double-clicked the SIT file that my system copied to the SheepShaver desktop. It expanded automatically and created a disk image for the game on the SheepShaver desktop.
5. I double-clicked the disk image on the SheepShaver desktop. This opened a window where I could play the game.
6. I double-clicked the game icon. The game opened, and seemed to play perfectly:https://imgur.com/a/NCpIoOL
Probably something is installed incorrectly in the system that you have. You might try mine, which needs no setup, no configuration, no effort at all. You simply double-click and run it. But read the documentation to find out the ways it makes things much easier to manage than standard SheepShaver setups. And it runs this game instantly.
Again, I didn't change or configure my system in any special way. I just dropped the SIT on my "Mac OS 9" app; double-clicked the SIT file copied to SheepShaver desktop; double-clicked the disk image that emerged from the SIT file; double-clicked the game icon; and played the game. All this probably took less than two percent of the time that you would need to spend on other methods offered in this thread.
The rules of the forum forbid me to post a direct link to my system, but it's easy to find with a web search.