I had the same issue a few weeks back, when I was away from home and I needed to remove the cassette to fit a new spoke. Like you I only had the adaptor and an adjustable spanner.
The first thing I tried worked a treat. Put the bike upside down and use the chain itself whilst resisting the turning of the cranks. I don’t think the wheel was in the dropouts, just somewhere neer so that I could bet a bit of the chain on the cogs. Little fiddly but very effective and a ride saver for me.
Good luck!
That was the right answer !!
You win this weeks official “Order of The Botch” – fashioned out of an old chrome moly frame and stem , with a black and red ribbon , to signify the oil and blood on the knuckles
Wear it with pride, young Jedi!
Most excellent – chuffed to bits