I had to make a little PCM storage unit out of an aluminum paint container. I used double sided tape to hold the Thrasher box to the container.
When I bought mine...I didnt like how it sat either. I had my thrasher, with my make shift PCM tray, and it stayed put for over a year and half beforeI up and sold it.
If you are able to double sided tape the lower section of the thrasher box to the top of the PCM....it will hold it. unless you are doing some HIGH G force cornering....it will stay put. I will try to find a pic of my Tray.