For me: Normal detergent, such as Tide, filled into a Nalgene 2 oz. Square Wide-Mouth Leakproof Bottle (found at e.g. Container Store for less than $2). Important to get the square ones! I am using them for years for a variety of liquids and never had any leakage. Hands-down the best bottles for this type of duty. Bring household latex gloves.
Never had an issue with TSA or other security checks with the strange blue color of the detergent.