Finally, an idea! I used a narrow knife and cut a slit an upside down T, leaving the tarp uncut below it where the snow will be.
Footlong straps and knots on each end through small slits on each side of the door slit. We can close it from inside now.
And a large flap over the door slit, held on by a strap running stitch to kee the wind off the door slit.
Realizable door opening.
It was nice to finally step into the tipi. And see it all again.