I have a glass topper for the wedding cake I am working on. It will be sitting on a 6 inch cake. What do you use to keep it from sinking into the cake and also hold it sturdy?
I had one of those glass toppers on my first wedding cake and I put dowels and cut a cardboard to set it on. It stood up fine on its on.
If you have flowers on the cake, you could always put some around the topper to keep it stable.
Does it have a flat bottom or a hollow bottom?
If a flat bottom, just put 2 or 3 dowel rods in the tier for the topper to sit on.
If it's a hollow bottm, and if it's REALLY heavy, I cut a circle from a clear plastic plate. I put dowels in the tier, place the cutout circle on the dowels, then the topper on the circle.
If it's a Wilton topper (the glass couple dancing), I've never had to use supports for those. They sit on a cake fine.