I did my first audax bivvy 2 nights ago, went very well. British Army gore tex bivvy bag, which I grimly thought may also double as a body bag as I lay down inside it. I picked a bit of a dry farmer's field which was very comfy (I think farmer's fields are good spots as the crops are draining loads of the water out of the earth). Crucially the weather was dry too, not much wind.
I used a go outdoors el cheapo 3/4 mat
https://www.gooutdoors.co.uk/15917939/oex-compact-3-4-2-5-self-inflating-sleeping-mat-15917939 , a travel pillow I got from my mum, and two of these cheap sleeping bags Decathlon used to do (I think they were £6 each or something silly), one inside the other.
I kept my clothes on (short sleeve base, merino leg and arm warmers, shorts and jersey) as well as my cap to keep my non-pillow side ear warm. I felt snug as a bug in a rug.
Not the world's most compact sleeping system but I had a really good quality lie down and had no feelings of sleep deprivation the next day. I think it was well worth the weight penalty.