The most common type of night guard or mouth guard that patients will get from their dentists is made of hard acrylic. They are sent to a laboratory so the patient would have models made of your teeth and most of these splints are made without looking at the system. Most of them are made without looking at how the jaw joint functions. Most of them look at just the tooth contact so they would make a splint to cover just one arch that could be the upper arch or the lower arch and they would then put them together. Fabricate the splint so it would be made in between jaw joints and it is just a protective covering against any occlusion, occluding that goes on with your teeth.
Easy squeezy. Not to mention you save a lot of money by going through an online night guard or mouth guard lab vs. paying a dentist. Hope this helps you feel confident about taking your own teeth impression. If you have ANY additional questions or concerns please feel free to ask or express in the comments section below. Thanks!