the slipping could be a factor of seat material and pattern as well... smooth vinyl with no pattern will prolly slip/slide you more than another material or even a pattern weaved into the vinyl. just a thought.
the seat bucket on my seat concepts seat on the Wee is too far back for me. same for a buddy of mine who bought the SC seat. he called SC and they couldn't change the location of the bucket since their foam comes pre-molded. may not be an issue with Russell...