I make a great one out of a pump up sprayer... got sick & tired of refilling the syringe every 2 min... when injecting several butts or briskets this makes quick work of em.
The small diameter needle keeps the juices from just leaking back out of the typical massive hole most injectors have.
You can get longer needles too, but this 1 3/4" one is my fav since it flips around backwards and stores inside the tip