After all these years, and with so many things revealed about TOS, if there had been some sort of model used for Mudd’s ship it would most likely be known. But it really does seem like it was merely some form of cheap photographic trickery to create an indistinct glowing blob on the screen.
The problem is that Mudd's Class-J ship appears more as an over-exposed (camera term) ship but not as a glowing blob. This is a glowing blob. Regarding to models from TOS, were there ever any bts photos of the Eymorg ship from "Spock's Brain" or knowledge of what happened to it? The TOS' FX information is a bit fragmentary so just not knowing much of Mudd's ship or how it was created or its disposition doesn't seem out of the ordinary, IMHO.