Modern slow cookers come in all sizes and colors with various bells and whistles, including timers and shut off mechanisms. They also come with a serious design flaw, that being the lack of a proper domed lid.
The first photo below depict a popular model Crock-Pot® sold far and wide in the United States and elsewhere. This pot is by Rival, a Sunbeam Products brand. Sunbeam owns the original Crock-Pot® trademark. Also known as a slow cooker, these pots were introduced to the market in the 1970s.
The next photo is of a Crock-Pot® from the 1980s with the original glass domed lid.
Older slow cookers such as this one have a convenience flaw, that being the cooking vessel cannot be removed to be cleaned.
However, what they do have is a the glass domed lid is that is missing from modern slow cookers. The domed lid is critical to a good slow cooking experience as it allows moisture to collect on the dome and drip back into the pot. This helps not only moisturize the food but aides in the tenderization process of meats. This is the same cooking principle used in clay-pot cooking using Moroccan clay pots known as tajines.
For a young enterprising man in this Year of our Lord 2019, should one wish to make a small fortune, it is recommended that one design and place in the market glass replacements for the various Crock-Pot® models being sold.
For the slow cooker chef, it is recommended that one peruse the pages of eBay in order to find a New Old Stock(NOS) model from which one may reclaim the glass dome lid. From experience, I can say with a great deal of certitude that it is well worth the endeavour.
Corningware sells a casserole dish through Target.com designed to be used in the oven that comes with a four quart glass lid. This lid should fit similar sized crock pots.