What doesn't seem to be represented in the discussion is that if you source and build your own epi pen, you might not have that initial discussion with your doctor and you certainly don't get any medical guidance leaflet with it.

Using an epi pen is a dangerous business and can cause considerable complications. It is absolutely not to be taken lightly and is literally for life-or-death situations. I fear that those making their own (or heaven forbid somebody else's) pen will not know this.

Source: my doctor when I bought my first epi pen (beekeeper).