These spacers are designed to prevent your weight plates from banging up against the mounting bolts on your plate storage pegs.
These fit typical storage pegs that measure 1.9" in diameter (slightly smaller than a barbell). This is the size used by Rogue Fitness.
This model will fit very loose on the REP Fitness storage horns, and will not fit at all on Titan Fitness products.
These will not fit over a barbell.
These are 30mm thick, or about 1-3/16". I've found that this is more than enough to clear most weight storage mounting bolts. If you would like a custom thickness, let me know in the comments and I can make any spacer thickness down to 15mm.