I just built a test run for under $20. Its worth the effort to build one out of cheap materials first to work out the bugs, and now I have a strictly light summer bag good to about 60dF.
Link to some pics below.
It actually looks better than the pics.
Overbag makes sense, although I dont think I would use silk inside. Next for me is .9 oz taffetta inside and ripstop out from thru hiker. Also an overbag needs to be a bit bigger, especially in the footbox. A liner could be smaller and lighter.
2.5 XP should be good to 50-55 unless you want it really warm then the heavy stuff. Really warm equals heavier too.
I dont think a drawsting footbox would be prefered for an overbag. It gets narrow fast towards the end and its a tad heavier.
I think My next one will be similar to the one in the link but with a closed footbox and velcro down 8" to the footbox.