How could he get it made up?
That would be impossible.
But, apart from this, just consider the strong internal evidence that there is as to the authenticity of the manuscript.

Now, in the first place, there is the description of Desolation Island, which is perfectly accurate.

But it is on his narrative beyond this that I lay chief stress.

I can prove that the statements here are corroborated by those of Captain Ross in his account of that great voyage from which he returned not very long ago." The doctor, who had been talking with much enthusiasm, paused here to take breath, and then went on: "I happen to know all about that voyage, for I read a full report of it just before we started, and you can see for yourselves whether this manuscript is credible or not.
"Captain James Clarke Ross was sent forth on his expedition in 1839.
On January 1, 1841, he passed the antarctic circle in 178 degrees east longitude.

On the 11th he discovered land in 70 degrees 41' south latitude, 172 degrees 36' east longitude.