Not amphib, but the Mooney Mite manual gear might be a good place to start for a simple, well-proven design.
"The most unusual feature of this little airplane, not counting its sensational rate of climb, is the neat manually-operated retractable landing gear. A long lever, similar to the parking brake on a pre-war auto, is mounted on the far right side of the cockpit. In the "down" position, this handle locks forward into a slot just above the right rudder pedal. To retract the gear, the pilot merely leans forward, grabs the handle, pulls down on it to release it from the slot and then brings it back about 18 inches into another slot just to the right of the seat. That's all there is to it! No hydraulic pump or electric motor- just dependable pilot-power."
--Pilot report by Don Downie, SKYWAYS, August, 1949
This article with pics and many more on the Mooney Mite Site.
Don Maxwell has some step-by-step photos of the manual gear in a larger Mooney as well.