Blow down tube is about $25. Drilling a hole in the body plug in the spare wheel well takes about 15 minutes. Bending the tube and mounting it to the bottle and sealing the hole where the tube goes into takes 15 minutes. All of this equals very little outlay in funds and time to keep things safe.
Search the net, if the tank needs to release and the burst disk lets go you want things to exit outside of the car and not in.
I have my bottle setup just like your's and it looks like you're using the same mounting bracket, so I know whats it takes to install the blow down tube = do it.