I'm not sure about why you say you open the mouth???
I have two techniques :
1) Send the tail/body (depends how big the snake) under an armpit, and hold the head/neck with the same hand that has the tail/body under that arm. You need to inject into the front third of the body so I use the other hand to inject. Amikacin burns so upon injection is when mine start to struggle.
2) Similar to the first technique. From a sitting position I hold the tail/body between my knees; one hand holds the head/neck the other injects.
I find that being fast is best, not negligently, just get it over with.
Also, I inject with needle at an angle maybe 45-60 degrees pointing towards the front to get under a scale. I hope that makes sense.
Good luck,
TomChambers