Grab a mediator, and you and the team mentor(s) in question sit down.
Things to talk about
:
--Personal issues with the design have no place.
You have to be able to back up the issue with something substantial. If you can't, then you have a choice to be quiet, or ruin the team (or at least your relationships).
--
Shoot the engineer and build the thing. If it's designed, build it as designed, unless there is a fatal flaw that must be fixed, or an improvement that nobody thought about before (and it better actually be an improvement!).
--If you didn't help with the design,
and you could have, you have no right to complain about the design. Sorry.
--If your design isn't chosen, you just have to live with it. You'll either be amazed, or you'll have the chance to say "I told you so". (Not directed to the OP, but to the mentors in question--I suspect that that was part of the problem.)
Also, you're a
team. If you're blaming ONE MEMBER for the team being behind, the law of pointing fingers must come into play, as you also are a team member. (The law of pointing fingers? You point a finger at someone, there are pointing right back at you.)
You could also print out
The Wrath of Abibarshim and post it, quite publicly. It'll provide some comic relief... but underline code item #3 and code item #10, particularly the time part, if you want to issue some pointed reminders.