Hi jasonone,
One way to achieve that with a spline-based method is “sketched” in the file below:
Scene file
https://www.amazon.com/clouddrive/share/Rg1Z3tvK3O7vv319xI2z0m62digQhQhSgr2hUDFVjba
However, I found it only useable if the amount of adjustability balances the complexity of the set up. In other words, it is most of the time to fragile for my taste.
The “box” modeling method would work, but I wouldn’t start out with a box in this case. The curve seems to be the most advanced part in the model. Start out with a small flat plane and bend it to the exact curve, then “extrude” or “extrude inner” from there. To get a more flat part is normally done by selecting the points in question and set the size in the Coordinates Panel to zero for the given axis.
(I just noticed while typing, that you answered already)
Anyway, I made a little clip to sketch the process, if anyone is interested.
https://www.amazon.com/clouddrive/share/MA4epan70AKeqS6fHNUrdKBO5m3Myjzss5iO0sCdyph
As a side note, points selections and a Restriction tag can keep the Bend Deformer active as modeling tool.
My best wishes