The bladder was custom made by Fuel Safe in Bend, OR.   It is foam filled, labeled blocks custom cut by them that you shove inside once the bladder is roughly in place.  Besides being one big baffle they also serve to keep the bladder more or less maintaining its shape from full to empty.  The bladder is 8 ft long and has a flat top.  When empty Fuel Safe said it would probably only collapse about an inch from the full condition.   This allows them to put access panels in the top that also serve to put the gauges, vent, fill fittings, and so on.  The gauges they used are capacitance by Centroid Products -- which just also happened to be the same gauges I had already put in the water tanks.  Capacitance gauges are continuous reading & more accurate (not the step reading of gauges like the Wema).  They have a rigid tube just like any level gauge but because the top only sinks a little bit that is no problem.  I calibrated the bladder, sensor and gauge so the effect of the (small) lowering of the gauge is automatically eliminated.  The bladder does take up some useable volume, 10% or so IIRC, but it is essential.
 
I built a snug-fitting "containment box" for the bladder out of 3/4" seaboard (a cheaper commercial equivalent of starboard).  You can see it in the picture.  It has top pieces that slide into place so the entire bladder, except for the part that rests on the curved hull, is enclosed by slippery seaboard.  The box is put together with machine screws and can be completely taken apart if ever necessary.  There is also a seaboard bottom to the box that rests on 1/2" seaboard cleats to keep it off the bilge and let bilge water freely drain under the box.  The bladder is not strapped down.  It is simply "contained" by the box.   The top of the box is "held down" by the floor "joists".   There is no way that bladder will ever go anywhere.   It is as secure as a metal tank bolted to the hull.   I can replace the foam if I ever need to, without removing the bladder -- simply remove the top seaboard covers in 2 places, open the access panels and go to it.   It will be messy but should be possible to do in a few hours, after the bladder is emptied.
 
Bladders are used in many applications, from race cars to jet fighters.   According to Fuel Safe the only possible issue with them would be with the foam (which sort of looks like water bed foam) -- namely whether or not additives that one might put in the diesel would eat it up over time.  It is safe for the diesel itself.   They said there should not be any problem with bio agents and that is all I ever put in my diesel.    The bladder is now going on its 10th year of operation and I have had a jar of diesel with the foam and bio agent in it on a shelf above my workbench at home and it (the foam) still looks the same as the day I put it there.
 
The bladder is what I refer to as my "storage tank".   I have a 25 gal aluminum "day tank" (about 20 gal working capacity) that the engine takes fuel from and a transfer pump to get fuel from the bladder (aka storage tank) to the day tank.  This transfer is automatic, controlled by a simple electronic circuit that I built, and has worked flawlessly for the 10 years.  There is a Racor between bladder and day tank, a second Racor between day tank and engine, and then the normal engine filter.  All filters are in series, so the fuel is triple filtered.
 
The box is not by any means leak-proof.  For one thing, like I said, there is no "box" along the curved part of the hull.   The "box" is only along the top, the *upper* sides, the ends and the bottom.   I consider the bladder better than a metal tank, and nobody every puts a metal tank inside a leak-proof box.  If the bladder ever developed a leak it would effectively just be going back to start -- the same issue we've all had with the OEM black iron tanks eventually leaking.   But I long ago quit worrying about the reliability of the bladder.   I wish everything else on the boat was as reliable.
 
Are there any issues?   Yes, but not with the bladder itself.   It's just that if you decide a containment box is necessary (as, like me, it appears you have) than that box is a lot of work.   That issue aside, I would do it again.  l  am very pleased and happy with the final result.
 
If you have specific questions, fire away.  I have a ton more pictures and can send a few more on specific topics if needed.
 
Steve
 
PS.  Do not make the automatic assumption that the existing wood work is inviolate.  As part of the fuel tank project I learned how to cut more openings in the floor and make them look ok.  They key is having the right tools, a Fein Multimaster and an assortment of Japanese hand saws.  Cut along a holly-teak edge in the case of the floor.