I'm a big fan of the little 4 oz. jars; my canning jar supply is entirely those plus wide-mouth (straight-sided) pints. I, too, struggled with a device to hold them stable in the water bath; for a while I used a silicone potholder on the bottom of the pot with a stainless steamer basket upside down over the 4 or 5 jars. I found a stainless steamer rack with small holes that works for the pints in the 10 qt pressure cooker-canner, and for the 4 oz jars in the 8-qt water bath pot.
The next few weeks are supposed to be rainy and steamy, so I'll be staying in and canning up farmer's market gleanings. Came back from the family reunion with home-grown squash and peppers that will be picallilli soon...