In Houston I would think a simple light blanket thrown over the tree with a 100 watt light bulb (can you still buy those in the US?) hung in the tree would be more than enough protection on the coldest nights.
Nice to know you are successful with your Meyer; grown in ground they are quite docile; and grown in the field as a crop, they are almost pets.
BTW my experience with Carrizo is that it IS trifoliate; I planted my first Meyers here in Guatemala on Carrizo; and it is how I trained my people to prune out suckers. I am not a big fan of that rootstock; it is quite susceptible to phytophera and does not tolerate wet roots; but in some cases it is okay.

Thanks for the feedback! That will be easy enough to try. We do get some nights in the mid 20s, but it generally pops back up quickly in the day.

From what I've read, the Carrizo is not quite as tolerant of our wet soils, and a bit more sensitive to hard freeze. Trifoliate is marketed here as different from carrizzo- and as being a bit more freeze hardy and better for our heavier wetter soils. But I have no personal knowledge- just trying to slowly learn, so I trust that you are correct! We are under quarantine for greening disease in our area, and most retailers seem to be selling from one large grower - at least right now, and they use mostly Carrizo except for dwarfs.
Thanks!