I have 8-ounce jars that are short – their tops are below the minimum water mark and putting 6 ounces of custard into each will be even lower. What’s the best way to elevate them so the top of the custard is the same as the water level?
I gather you do not trust the lids and this is why you want them elevated. It is more usual to want them submerged is that they are surrounded by the cooking medium.
Turn a bowl or a cup upside down (it will need to be full of water) and sit a wire rack (cake rack?) on top to form a platform.