On our professional shows we use short wooden stakes to secure our cakes in place.
Use one stake for each of the cakes in your display.
Hammer the stake into soft ground and place the cake behind it (so the stake is between the cake and the audience).
Secure the firework to the stake with gaffa tape (but don’t tape over the fuse!)
This makes it safer – far less likely to bounce around or fall over.
If you are firing your display in rain, you can even put the whole cake in a thin plastic bag (like a pedal bin liner) to keep it dry. The first shots will rip the top off the bag and the whole firework will still work properly.