When we cook our cholent (overnight stew), it sometimes dries out on top. For those who do not relish this part, I am looking for a stainless steel insert to my saucepan that will keep the solid ingredients below a liquid layer. Can anyone recommend a product please?  
Using a drop-lid or otoshibuta is commonly used for Japanese simmered dishes. Take a look at the lids shown on this site: the metal ones are more or less the shape you described, albeit without a lip. Although Japanese drop-lids typically sit above all the food and liquid, I see no reason why they could not be slightly submerged. Even sitting on top of the whole stew, a drop lid would significantly decrease evaporation. The French tradition uses a piece of parchment paper instead. Like a drop-lid, it simply sits on top of the braise and prevents evaporation similarly. This article provides instructions for using one. 