OK. Bad news I'm afraid. I tried a Protank head in my Aspire tank and it doesn't fit. I thought the chimney on the head would fit inside the chimney section in the tank, but it was too big. The post I read must have been referring to a different tank. I read it and didn't bookmark it so I can't go back and check. It was talking about putting a silicone cup over the bottom of the chimney section of the tank, then pushing the chimney section of the protank head up through the cup to get the seal. If I come across it again I'll let you know.
There's different thicknesses to the Protank chimneys. I do use the thinner ones as you described in my Subtank mini.