Making the battim (the leather boxes which house the tefillin parchments) of Shel Yad in relatively simple since it’s just a simple compartment that’s smooth on the outside.
On the other hand, the Shel Rosh not only has a letter ש painstakingly embossed on both sides, but also it is formed of four separate but unified boxes.
Types of tefillin
The terms Tefillin Peshutim Mehudarim, Tefillin Gasssot, Tefillin Gassot Prudot and Miksheh refer to how the battim, particularly the Shel Rosh, are constructed. Because the materials, the tools and the level of craftsmanship used to make Tefillin Gassot are more expensive, expect to pay significantly more for Tefillin Gassot than for Tefillin Peshutim Mehudarim.
Also keep in mind that tefillin makers almost invariably insert mediocre parchments in Tefillin Peshutim Mehudarim; the more expensive the battim, the better the parchments on the inside will be.
Tefillin must be made of a single piece of leather (“ohr echad“). The big question which then arises is whether several pieces of leather glued together are considered ohr echad.
The problem used to be especially acute before the advent of Tefillin Gassot, when Tefillin Dakkot were the norm. Because Tefillin Dakkot are made of thinner hide, glue had to be applied in the crevices between the four compartments of the Shel Rosh to insure it kept a square shape over time. The problem is that from a halachic standpoint, if you say glue unifies the compartments, you have now created a single compartment instead of four separtment compartments, as required.
(The Vilna Gaon, upon coming across someone whose Shel Rosh was constructed with glue, would quip, “Why are you wearing a Shel Yad on your head?”)
On the other hand, if you use no glue (a method which today is referred to as Tefillin Prudot), the Shel Rosh is liable to lose its square shape. (Because today’s Tefillin Prudot are made of thick leather, they maintain their shape much better.)
The Chassam Sofer argued that you can’t have it both ways: You can’t make Tefillin Peshutim Mehudarim using glue to unify several pieces of leather — and then use glue to hold the compartments together, now saying glue does not unify them.
Various Achronim hold that in fact the four compartments of the Shel Rosh do not have to be truly separate, rather you only need slits visible from the outside.
The Chassam Sofer wrote that if the tefillin maker does not have the intention to glue the compartments together, but rather uses a small amount of glue just to prevent them from separating, then they would be considered four separate and distinct compartments.