Over the years, I have used slotted spoons to hold down the shell, a shot glass or small filled water bottle. All these can leave marks and possibly knock of fine waxing.
At the 2019, PysankyUSA Retreat, in Wilkes Barre, Pa., a smart idea was found in our 'goody bags'. It was a handmade dipper, a clever safe way to hold down the shell in the dye. I made this 'how-to' slide show so you can make your own.
I found my wire at ACE Hardware but also Amazon.com