DIY projects are great fun and teaches a lot on your camera equipment. I did something similar for Phase One and Sinar. This is medium format tuning! Hopefully more people will show their inventions.
The bellows is made of neoprene by hand glueing seems together. Little tricky but very strong and durable construction. Besides the Sinar looks so dated with original wideangle bellows. The frames were stolen from a used wideangle bellows.
It works really well and cost is marginal. I basically had all parts. The plate is made of a lensboard from an old 4x5 lens which was totally worthless anyhow. The PhaseOne adapter came with my Horseman SWD2. The bellows is made of neoprene which you can buy from many diving stores. Cost about 10 eur + glue another 10 eur.
The plate itself is slightly machined to fit the PhaseOne mount.
It is possible to use many kinds of lenses. I bought a used Sinar Digital lens from Ebay with DB mount and that works really well.
My 2 cents,
Patrik