Little purple caterpillar

We had a heat wave last week. 37.9 degrees in October! It got me thinking about summer sleeping. In our dink days we put separate air conditioning units in to our bedroom and the living room as our roof was too low for ducted. The house gets hot and the house gets cold. In winter Moonshine sleeps in a fleece sleeping bag and flannelette sheets. (Home made when I wondered if Rainbow woke at night because she was cold - No it didn't help her sleep!) In summer I always slept Rainbow in a singlet and nappy. Moonshine is a little different. She likes pressure. She was a baby who appreciated being swaddled and now sleeps in a sleeping bag and firmly tucked in. Also the main way we help is by holding her in her bed with our whole arm resting along her body.
So I re purposed some size 2 singlets (a pack of 5 brought for Rainbow but never used) extended with a panel fabric from my stash. Closed at the base with a drawstring.

(She can't lie still in it.

The whole time I fed her prior to her sleep she was jumping around,

pushing her feet off the fabric)

(Look she's doing the lying like a frog that babies do! So cute!)

Moonshine is wearing it for the second time tonight. The first time she wore it was a day sleep and she put herself to sleep without help for the first time in weeks.

I'm thinking of sewing 4 more but I need to iron out some problems.
I had hoped that I could just pull it on over her head but found the panel doesn't fit over her head even though I had

1. Sewed the panel (a very stretchy knit) so that the stretch ran across, the same direction as the singlet's stretch.
2. Used a fine zigzag stitch on the machine recommended for sewing knits.

This lack of stretch at this join also means I can't do a nappy change without undressing her as I can't pull the bottom up over her nappy.

Putting it on feet first is fine and hopefully once I stop feeding her all night I can stop night time nappy changes without dealing with wet sheets. It also means I could just end it with a seam.

I'm only concerned though that it will stretch the neckline. So how can I change that join. I don't want to gather the panel as I want it to be firm fitting. I did wonder if I could sew the bottom of the singlet and the panel on either side of a wide piece of elastic. It would look ugly but would it work. Any ideas?