I just finished making a prototype that I've been working on the whole time that I was gone. It mainly uses a G2 and a HGG barrel, but most importantly it's retractable.
Edit:
OH! and yes!!! it is very durable!!! i am reluctant to say it because i might get pwned, yet i'll say it anyways because i know its true... I think it is even more durable than a RSVP in all areas except for the tip, which sometimes comes loose, but that can be fixed if you use a different method than i do to keep the tip on the pen, which is to screw it on.
Here's a pic with it un-retracted(is that the right word?) along with an MX that also doesn't have an insert so everyone can get a gist of what the size of it is.

And this is a pic of the ink pushed out of the pen with an MX again for comparison.

Tell me what you think!