+1 on the poncho liner UQ. That is what we used our first year. You can add and subtract more insulation to this foundation as needed. Doing the no sew leaves your options open to when you have a proper UQ your poncho liner can still be used for other things; IE overcover, undercover, extra UQ and so on.
+1 also on the conversion of a down sleeping bag, if you have one, or even a child's sleeping bag if weight isn't an issue. There are ways around the upfront expense of a proper UQ to get you through for now.