[nerdponx]

"-is" is typical 3rd declension. If it's not neuter (how do you pick a gender for loanwords in Romance languages?), it'd be Redes. If it's neuter it'd be Reda as you said.

Or maybe it'd follow the -eris pattern and be Rederes or Redera.

[jedberg]

I thought 3rd declension is just 's' for the nominative singular. So we'd need to know what the stem is.

[nerdponx]

The 3rd declension nominative singular can be anything. "Red-" and "Reder-" were my two guesses at reasonable stems.

I wouldn't be surprised if Neo-Latin hobbyists had rules for things like this.