Looks awesome, nice work. I like that color. You're right about turning one into an underquilt. I did that as my first prototype and it was fast and easy. I used a North Face Allegheny that I got cheap at REI and cut it to fit my HH. I still use it in the summer and it works great. Here's a link to a post I made about it on WhiteBlaze, if it's any help.
Very long post