Silicone o-rings come in a variety of widths. The "standard" size that you see people using is 1/8" width. I myself have many o-rings (of varying inner diameters) that are 3/16" width. Which, to be frank, are thick as hell.

An example: https://www.amazon.com/gp/product/B000FMUUM6

The seller, Mr O-Ring, has every size you could possibly need (no, I'm not affiliated, I was just able to find every size needed to complete my stack).