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get a cheap standard or maxi sized resin cage that comes with multiple size rings because when you're first trying out it's going to take some trial and error even when doing all the measurements well.
i found this one comfortable in small, but had trained to get there. root around and find a maxi size if you thin theyre at all above average.
FYJENNICC V3 Chastity Device Cage Male Briefs Medical Grade Resin Breathable Men's Underwear(Purple,Stand) https://www.amazon.com/dp/B087QGW9MX/ref=cm_sw_r_cp_api_glt_i_Z0YSJ0FP4RN2NC3647C5?psc=1
FYJENNICC V3 Chastity Device Cage... https://www.amazon.com/dp/B087QGW9MX?ref=ppx_pop_mob_ap_share