I put a Grolsh bottle gasget in the bottom of my test jar to act as a cushion in case the proofing hydrometer hits hard or slips from my wet hands. I guess any plumbing type plastic or rubber washer would work fine if the bottom of the alcometer is rounded like mine. With a pointed bottom you would need a few washers or some sort of pad with no center hole. Just make sure to reinsert it if you pour it out after your reading. I test a lot during a run by putting a funnel on top of the test jar and catch the output and then check the reading when nearly full and pour the jars contents into a storage container, leaving the alcometer in the jar. If, during pouring, I tip the jar too much the alcometer will slide near the top and then smack the bottom when you set it up vertically. If the washer is in place I feel a bump when the alcometer hits bottom reminding me to take it easy. Without the washer I feel a crunch which reminds me to buy another alcometer. Good Luck