I recently purchased a Hursch f6.6 focal reducer which just came in a cardboard case type box with a cleaning cloth. I also have a Meade f3.3 focal reducer but this came in a plastic bolt case.
I put a wanted ad for a plastic focal reducer bolt case on astrobuysell and I also called into ScopesnSkies to see if they had one, but they did not.
They are the half size cake decoration boxes that contain sugar stars, silver balls etc. from companies such as Supercook and Dr Oetker. So I removed the outer packaging and cleaned the inside out.
I then cut out some thin foam and glued it into the case and then placed my focal reducer in and it fitted perfectly. The focal reducer does only fit one way up though, but is now airtight.