What hammock do you have?
Heavy 4wd camping or hiking weight or ultralite?
I'll share my hammock journey so far. I'm in same area as you.
Coming from 4wd camping to a HH required an UQ so i took an old synthetic -5 sleeping bag some shock cord and office binder clips and bobs your aunty cheapskate redneck UQ.
It worked down to -2°c out at Canarvon Gorge on a hiking trip.
Took ages to set up properly.
My son still uses this set up.
I upgraded to a Warbonnet blackbird XLC with matching 20°f(-6°c) Warbonnet wooki UQ.
Set up time of the UQ is now 10 seconds and so comfortable and warm. Bit the bullet with a diamond back TQ 0°f(-17°c). I'd recommend ordering 0°f to accommodate future cold climate hangs. As you know US-Aus exchange rate makes US cottage products double in price by the time it gets to your hands.
I took 6 Costco down quilts (Ipswich Costco) and made a TQ and UQ for my daughters Warbonnet Eldorado. I followed plans found on this forum.
700 fillpower down all over the room no matter how careful i was. TQ turned out a treat. Packs down small and she's used it to -1°c so far. Cost was about $140 but quality looked great off the sewing machine sadly ordinary 1 year later. The UQ needs more work as she has cold spots.
My recommendation for you is use a cheap sleeping bag redneck style and save for whichever Under Quilt that tickles your fancy.

Undercling Mike on bushwalk.com has a good reputation here in OZ for Top Quilt.