Basically, you need a slide with holes placed in it so that when you move it up it allows air through to both reeds and when moved down air is allowed through to only one of the pair or the other way round?). This can be operated by a stop or by a switch depending how handy you are are designing and building linkages. I'm not sure whether or not the reed chambers on the pokerwork are designed to allow this ie can you block off the air to just on of the pair of reeds - you might need to put some sort of wall between each reed.

The problem with the fingers slipping into holes is usually overcome by putting something under the finger board. On my one-row I used carpet. This also cuts out a lot of the clatter.