Hmm, OK, I do have an idea here.
So, we could have Shirou summon Toshi who, in turn, summons Caster. Caster then manages to use Rule Breaker to capture Rider and, appreciative of Shirou's kindness, decides to give him a reward by bringing a de-wormed, naked and bound Sakura to him as a gift, whilst keeping Rider for her and Toshi to use (obviously, Toshi will insist on consent before anything is done to her). Sakura, of course, does not object one bit, once she finds out who she's being handed to....