for 38 degrees, on the cheap you could make a Poncho Liner UQ. But you'd have to add some insulation to it... weight? about 20oz by the time you're done. a pad (ok, you'd need 2) would be the lighest/cheapest option.

Walmart has a wide (24") waffle pad that would probably work great for you.

The reason you need a wide pad is for your shoulders. The hammock wraps around you, compressing insulation on your shoulders and hips. If you took a pad, and laid another short piece that's wider for your shoulders, you could avoid that situation...