If it's any help, the realistic skin and body part models for medical
training (different injection types including IV, intubation, and even
more invasive procedures) are fairly recent. Medical students and nurses
used to learn to do injections starting with oranges, and then, on one
fine day, they would be told to practice on each other. The clinical lab
at Georgetown would usually get advance warning and watch the
white-faced students. A few would faint.

Seriously, I wonder if some vets might invest in the training aids for
injection, for teaching their patients. Failing that, I'm trying to
think of something to improvise practicing a subcutaneous injection as
you do for fluids.  

Oranges aren't bad for learning IVs, but they deal with taut skin. One
of the tricks of drawing blood or starting an IV is expecting the skin
resistance, and then the "pop" as you get through the skin and into the
vein. IVs are harder than SCs, because the vein can roll from side to
side, collapse, or you can go through it.  Still, I'd recommend you try
practicing with an orange, or perhaps a tomato, to get a little feel for
the technique.

Incidentally, the technique of giving cats fluids this way was once
widely used with people. For an assortment of reasons, IVs weren't
routine until roughly WWII and afterwards. IVs are more efficient with
people, but take more skill.  The technique used for subcutaneous fluids
is called hypodermoclysis, and may be almost as efficient for furpeople
with loose skin.  Still, you need an IV in a cat if you need to get
controlled amounts of fluid in quickly.