thanks for the suggestion. i got another suggestion, which seems obvious now
Buy an additional 49 to 52mm step-up ring...it has a m-49mm in one end and a f-52mm in the other.
A step-down ring has a m-52mm in one end and a f-49mm in the other.
Screw step-down to step-up to step-down...f-49 to m-49 and f-52 to m-52.