I don't do it that way; I use 6 gallon pails and just 'top them up'
once it's mostly done. I have at least a week of active fermentation
and it takes a while - usually at least 2 weeks total for the cap to
totally sink. By then I'm pretty sure I have as much out of those
skins as I am going to get. Once the wine is still for the most part
I start combining pails and let the residual CO2 cover them. I do lid
the pails the whole time.

Your way sounds OK, I just don't have gas available. What some people
do is just lay a few layers of plastic sheeting on top of the wine.

Joe