re: #504 Cato the Elder

That would be “doofi”.

Doofus is fourth declension, not second. So it would be:
Nominative: doofus (s) doofus (pl)
Genitive: doofus (s) doofuum (pl)
Dative: doofui, doofu (s) doofibus, (doofubus) (pl)
Accusative: doofum (s) doofus (pl)
Ablative: doofu (s) doofibus, (doofubus) (pl)
Locative: doofi (s) doofibus, (doofubus) (pl)
Vocative: doofus (s) doofus (pl)
A common mistake, but being one, I would know.