This Lysol toilet bowl cleaner sold at Walmart has 9.5% HCL ... "There is no plastic case, so they simply dissolve." Just stamp the applicator under the rim and release. It couldn't be easier.

Best Gel Toilet Cleaner: Power House Toilet Cleaning Gel Dollar Tree customers say that applying a small flower-shaped stamp of Power House Toilet Cleaning Gel "really keeps the bowl fresh and clean." ...