A see through gum shield is a must for sparring.
To mould- immerse in boiling hot water for 10 seconds, shake out any water and bite down into gum shield whilst moudling it to your teeth with your fingers, and sucking. Repeat this method until it stays on your top teeth when your mouth is open.
Mr Whitlock's pro tip- if you have a strong gag relfelx, saw off a couple of centimetres from each side with a bread knife until you no longer gag. But don't cut off too much and expose too mant teeth.