Figuring out how to weigh down the tops of small batches of miso is a bit of a pain. Of course, a compacted medium is best for the fermentation process. Here's a straightforward method for accomplishing the compression with standard canning jar lids and a wide mouth jar.
Compact the miso as you would normally to get as much air out as possible. Fill until the compressed paste line is as seen above. Check the level by placing a regular sized lid that just fits inside the wide mouth jar with the lid against the miso and ring on the top. The ring must be above the top of the glass to create the surface pressure.
Bag the lid and ring to prevent rusting from the salty environment. Make sure the lid is still outside of the ring. If you use a plastic top, you can skip this step.
Put two layers of plastic wrap over the mouth of the jar with at least one inch of excess all around.
Compress the bagged lid into the jar to force the plastic wrap to squeeze around the jar lid and ring.
Gather all of the plastic and bring it into the center of the lid ring then put the wide mouth lid over it. Push the lid down onto the jar as you tighten the ring until it just engages and compresses the miso. If you can't engage the ring, you will probably need to remove a small amount of the paste.
When you're done, the top must not be sealed. There has to be a gap for the future carbon dioxide created to escape or the vessel will pressurize and potentially explode. Also, don't forget that the tamari (liquid) will start coming out of the jar so make sure you store the miso to account for that.
Coming up with a simple solution with readily available parts that don't need modification is pretty great. Sometimes the answer is this easy.
As always, please share your discoveries to keep the ideas bouncing.