Since it has been mentioned twice, an undressed altar is just a table. No need to acknowledge it in any way. It is when you walk in front of a tabernacle with the host in it that you kneel and persignate. It so happens that many churches have their tabernacle behind the altar, so you see people reverencing the altar, but it is really the host they reverence. Still, many people just kneel in front of an altar that doesn’t have a tabernacle behind, just out of habit or imitation.
Interesting.
I was confirmed in an ELCA church, this exact church right here in the Chicago suburbs within walking distance of my house, which entailed two years of Saturday morning confirmation class during seventh and eighth grade (1967-1969), and I never heard Word One about crossing oneself. In order to pass said confirmation classes, I memorized reams of excerpts from Martin Luther, memorized the worship service liturgy, wrote a two-page paper analyzing one sermon of my choice, and spent a hellish two weeks at the coyly named “Campfirmation” the final August, along with kids from three other churches, up at Lake Geneva, which entailed 14 days of Bible study and lectures–and not a single word throughout the entire proceedings about crossing oneself.
And after finally being confirmed as a member that fall, and attending services there as an official “adult” until the tenth grade when we moved away, I never crossed myself, nor ever saw anybody else doing it.
And I had a best friend who lived across the street from me who was a serious Catholic, meatless Fridays, holy water on the wall of her bedroom and everything, and if anybody at MY church had been crossing themselves, I’d have noticed. Because crossing yourself was a “Catholic” thing to us on the block. And, as we were firmly told during all those Saturday morning confirmation classes, WE were “Protestant” (and moreover, “speshul Martin Luther Protestants” 
), which was “the opposite” of Catholic.
I do recall learning that pretty much anyplace can be an altar. Any table, flat rock (or rounded), the hood of a car, or anyplace else the host is. Priests have given communion in the mud on a battlefield.
Also, the wafer, or whatever (a hunk of bread) is used in the sacrament, is just that until blessed by the priest.