This has been discussed many times over the years and I am not sure anyone has come up with a recipe.
Keep in mind that the yogurt is a perishable food - no matter what you mix with it it will need to be frozen unless you can use up the entire batch in a very short time.
Adding corn starch may thicken the yogurt but - it also adds a lot of phosphorus - defeating the goal of offering homemade yogies for the calcium it provides.
You can use plain or fruit flavored yogurt as a treat by allowing your glider to lick it off your finger. Just remember the 3 lick rule - 3 licks then reload your finger. If your finger is licked clean your glider might bite to make more "sap" flow as they would if they were licking sap from a branch of a tree.
For the small number of yogies you will offer your glider as a treat now and then - there is no harm in buying the small ones sold in pet stores.