This question has been on my mind all day since with the altars being dysfunctional, I'm kinda stuck with a forest of trees
Anyway, let me think this over step-by-step.
First off, to the best of my knowledge, a fenth altar is a device that "decomposes" the creature back to its original form, after which the owner gets all of the energy and knowledge of the creature: heat, fenths, stats, and magic principles depending on which altar you used.
Reading the adventure log, we know that the altars are now somehow connected ("...reunited the bonds that once bound each alter together"). And recently, their energy is being channeled elsewhere ("...the shades begun to channel the energy from the alter in Necrovion ...).
In my opinion, the interesting part here is the implication that in the past the altars have been connected like this.
Was being connected their original state, and they somehow got torn apart later? Or, is it simply implying that such an event (the altars getting connected) has happened before?
From the way it is worded my guess is the former. Either way, we can conclude that the altars being connected to each other isn't a requirement for them to work (they did just fine before), in fact the whole reason the shades were able to mess with them like this is said connection.
Therefore, for an altar to work under the current conditions, it would have to be built in a way that it is impossible to connect to the others - kinda how Aia has suggested above, although I'm not entirely sure about the heat vein explanation.
That being said, the heat vein theory could be confirmed if we did a bit of digging around the currently existing altars. My guess is that IF they are indeed connected to heat veins, they should have parts that go way underground. Assuming that the heat veins are way below the surface.
If heat veins were close to the surface, altars could have been built anywhere. But, their placement seems to have some pattern.
From what I saw, they are always built on flat land, never on mountains. So, close to ground level.
And the biggest giveaway is Golemus, which mainly consists of mountains, yet the altar is built on a shore of all places, inside a cave.
This makes me think that making our new altar float may not be enough in itself (what if heat just get "sucked down" into the veins if they are close enough), but we should make effort to build it on the highest point available.
Of course, this is all assuming that the altars are indeed connected by some kind of underground energy network. If they are connected by other means, like telepathically, or maybe on another plane of existence, we will have a harder time stopping the connection on the existing ones, but on the other hand, if we build a new one it is very unlikely that it will be "plugged in" to the network just by the fact that it is an altar.
As for how it should look, it doesn't seem to matter that much. All of them seem to have some place where you can "place" the creature, and a form of killing mechanism. As long as we have those, it should work in my opinion.
Well, that's enough brainstorming for now.