it needs to be held down. i would ratchet strap it down for now. if you brake hard for some reason there is nothing (other then the cables) stopping the battery flying forward where it can short on the bonnet or somewhere and cause a fire
As stated the standard brace is can be real pita to deal with. What I've done on my N/A is bolt a hat shaped piece of ally bar to the bottom of the battery tray then drill & tap the ally then simply bolt the standard clamp to this. The top clamp could easily be replaced with a length of angle.