The following material is taken from hasofer.com and may answer most of your questions.
What you are referring to as "Gassos Prudos" sounds like thick skins that are not from a single skin but are glued together. This would be by definition, less mehudar.
Minimally kosher or mehudar? It's your choice!
Jewish law recognizes 4 levels of kashrut. An item that meets the minimum standards of halacha is called kosher l'hatchila l'bracha, kosher or fitting for a person to recite the appropriate blessing and use it to perform a mitzvah. If the item doesn't meet even these standards due to its containing halachically-condoned errors or poor quality it may be considered kosher b'diavad, minimally kosher according to halacha: permissible for use but only when no item of higher quality is available. Pasul refers to an item which doesn't meet even the lowest of standards and which is not suitable for use for a mitzvah.
The highest level of kashrut is mehudar, beautiful or enhanced. The Torah teaches us that in addition to fulfilling the mitzvot we must also beautify them to the best of our abilities. This concept applies equally to external aspects of the mitzvot that we can see as well as to hidden or unknowable ones. The technical expertise and quality of the sofer's craftsmanship as seen in his writing and the perfect square of the tefillin batim are visible to the human eye. But more important are his yirat shamayim, personal integrity, and the breadth and depth of his knowledge of the laws of STa"M, as emphasized by the Chofetz Chaim's words quoted above. Going one step further, the Talmud in tractate Succah teaches us that even the kulmus, the sofer's feather, should be beautiful. Some sofrim store their kulmus in a silver stand for this reason.
It is crucial to know the source of your tefillin and mezuzot, who wrote and examined them, and who put them together. The sofer, examiner, and batim maker must all be G-d fearing individuals, professionally trained and properly certified by recognized Rabbinical authorities.
Tefillin components: Parshiot and Batim
The two most important components of your tefillin are the parshiot, parchments, and the batim, housings. These determine 90% of the price of your tefillin.
HaSOFER tefillin are divided into two types according to the tefillin batim, housings:
Gassot tefillin batim, also called ohr echad, are made of a single piece of thick high grade leather that is stretched over metal plates and folded and pressed to form the familiar perfectly square tefillin shape. These are the very best type of tefillin batim available, both in their adherence to halacha and in their longevity. Exclusively at HaSOFER, the Hebrew letter shin on both sides of the tefillin shel rosh is manually pulled out of the leather while it is still moist, using a unique pliers-like tool we designed ourselves. They are made with a natural drying process that lasts 2 years and have a lifetime of up to 60 years when properly maintained. Gassot tefillin are an excellent long-term investment if it’s within your budget.
Tefillin p'shutim mehudarim dakot batim are also made from a single piece of high grade but thinner leather. The required perfectly square tefillin shape is produced by folding and gluing the leather. Average lifespan of HaSOFER'S tefillin p'shutim mehudarim is seven to ten years, but this can be extended to up to twenty years when properly maintained.
Two other types of tefillin batim, which HaSOFER doesn't carry, are p'shutim and dakot. Tefillin p'shutim are made by gluing together many individual pieces of thin leather. These generally last only three to five years, after which the pieces begin to separate and the tefillin p'shutim lose their required square shape making them invalid for use. Dakot tefillin are made from a single piece of thin leather stretched over a frame and folded into shape. When made in the ideal manner they can last up to twenty years. Dakot tefillin are very difficult to make and are not currently manufactured at a kashrut level that meets the standards of HaSOFER.