Realistically, a properly-working bicameral legislature would have 15 members total, not just 15 in the upper house. That's just me though.
Also, on further thought I would not participate in a universal system, due to the ridiculous inactivity of a universal lower house. I came up with an alternate idea for a bicameral congress which I guess I'll keep around for the fifth convention.
We can always change the proposals by voting and motions. Feel free to propose it in whichever of the current models you think it could fit best. There has been no discussion in one of the threads (I forget which one off the top of my head), so perhaps an idea to change it up a bit would get it moving.
Worth a try.