I used mason jars for a few years but recently switched to a 7 cavity silicon mold. The mason jars worked well, but were time consuming to clean and the eggs sometimes were difficult to get out. The bands and lids also need to be replaced after a few times due to rust and seals wearing out. I also had one break once - my fault for having the lid fastened too tight - and that was no fun.
Since the mold is not very tall, you need to build up a platform of some sort in the water bath so that the Anova is properly submerged, but the water stays below the level of the mold's lid. I use an inverted bowl.
I bought the egg mold for use in my instant pot, but did not like the results at all. Sous vide is the way to go for egg bites.
The egg mold also works great for making large ice cubes for cocktails!