What size double sided suction cup from lustarts have you used on your m/m flared chance?

Hello all.

After getting to know my stallion i've decided that i want to try some new positions that require a suction cup to pull off.

Measuring the base of mine i think i could just fit a large-large suction cup from lustarts but i am a fraction worried that the edges will cause it to pressurise due to a fractional overlap or being on the border of.

What sizes has everyone used to pin their boy to a wall (toy side size - wall side size) and how successful were they are holding him there?