I know I've said this before, but it bears repeating: no matter where you mount your extinguisher, it helps with muscle memory if you occasionally practice using it. Sit in the driver's seat as normal, then go through the steps you will need to do to extinguish an engine fire:
Pull engine lid control;
Open/unlock extinguisher bracket;
Open engine lid, engage extinguisher. All portable extinguishers I know of have a pin that needs to be pulled prior to activation.
Be aware that you won't be functioning on a "Jeopardy contestant" level if/when you have a fire in your car. In fact, you may be struck stupid. When you practice, it will help you consider the process, including how you will keep a hot engine lid open while you engage the extinguisher.
A little practice may persuade you to consider a fixed, automatic device, hopefully one that is UL-approved. You may even want to keep an old pair of leather gloves near the extinguisher to open the deck lid.