A few important points come to mind from what you write.
1} Dermovate sounds good. Put it on the tight part, and the tight part only
- you will be able to see and identify the tight part when you pull the foreskin back.
2} Whether or not it is "BXO" you have, if there is any tendency to redness or scaling or cracking as you describe, use some antifungal such as Daktarin® also
on the tight part and around it.
3} Whatever pharmaceuticals you use, apply only enough to coat the skin and disappear.
you have a problem with the foreskin, do not use soap
- whether it is "Dove" or whatever. Use water only on the foreskin and glans. It may be less important what you use nearby whilst your foreskin is covering your penis. This is especially
important if there is any tendency to redness or scaling or cracking as you describe.
5} It seems you can pull your foreskin back fine when flaccid (soft) and the limitation is when erect. Certainly you need to stretch, and the stretch needs to be (eventually) to a size wider than your erection. The basic
means of doing this is to get two fingertips inside and pull apart.
6} Your modification of a speculum is commendable, but the reference to "2 prongs on the end" makes me suspect a slight misunderstanding in engineering. Any "prongs" must be as wide
as possible, such as to spread the force over as great an area as possible. Making them too narrow is a likely cause of discomfort.
The tips of the "beak" of the device as modified need to be wide enough to only just fit in the opening of your foreskin (as you stretch it to permit this) before
you open it. They also must be very smooth and well rounded
- as well as filing them smooth and using fine sandpaper, you can "heat flash" the plastic very carefully (i.e., very briefly, repeat as necessary after checking each time) with a hot air gun to make it go clear again.
7} The "bushing" or "grommet" shape requires some explanation. I am having some trouble with my support site so that I cannot at present even post a drawing, but the "tube" needs to be wide, but short, and have a "groove" or "slot" around the middle - with very bluntly rounded edges as before - for the tightness of the foreskin to sit in place. In regard to soreness, the ability to wear a device all day if not for the whole 24 hours, is far
more important than the degree of stretch, so you have perhaps been making it too tight a fit. Make one according to this suggestion, so that it comfortably holds the foreskin open, and try that awhile.
(Here's an example - if it shows up - of a grommet with one "cheek":
If you can visualise one with two "cheeks", one on each "end", with the fairly wide "groove" between, you might get the idea.)
8} Lose the Austin Powers penis pump.