18a filter holder

The 18a/20a filter holders are made from 2 Al washers, sandwiched together. The inner hole of each washer has a lip on it, just slightly smaller in diameter than the 18a/20a filters, and recessed deep enough so that the filter is flush with the top of the washer when the filter is inserted.

The washers are 1.65 mm thick.
The depth to the lip is about 0.94 mm. (it is at least as deep as the filters).
The second washer is identical and directly in contact with the other washer, so that it acts as a retaining plate.

Together the two washers completely occupy the 1" filter hole, so that the retaining plate on the top of the filter wheel is in contact with the top washer. The retaining plate is about 0.8-0.9mm thick.

The filter wheel itself has at least 3mm of clearance to the SDF assembly lid.