For ‘portmanteau’ Vikipedio gives ‘kofrovorto’, which I take to be an acceptable translation of the linguistic sense of ‘portmanteau’. Vikivortaro gives ‘kunfandiĝo’, which seems to me to reply too much on context to distinguish it from a process, such as the creation of an alloy. Yes a portmanteau is, analogously, an alloy, but in that case why not just give ‘alojo’ this additional sense? On the other hand, there is some support for Vikivortaro’s choice, in the form of this note from Merriam-Webster’s entry for ‘portmanteau’:
“The technical (and simpler) term for such words is blend.”

Surprisingly, neither Wells not Benson offers anything for the linguistic sense of ‘portmanteau’. ReVo and Sona offer nothing at all.

So, kudos to Vikipedio!