I'm curious about Nether Portal mechanics and wonder if one of you smart people already figured it out.
I built a structure in the middle of a lava lake and I'm wondering what I can do to reduce or eliminate the likelihood of someone accidentally creating an end Portal at precisely the right location in the over world to cause them to pop up right in my space.
What is the minimum height required for a Portal to spawn?
Will portals spawn on user placed blocks?
Are there blocks that portals will not spawn on?
Other considerations?

i know that portals wont spawn on:

• fire
• slabs
• signs
• most intangible blocks
• blocks with functions like chests, enderchests, enchanting tables, though im not sure about crafting tables and furnaces.

All of these are from tests i did back in version 1.3.2 so im not sure if this is all still true (except for fire that one i know works for sure)

All i can suggest is linking your portals, not sure if it will stop anyone from finding you though

@BluePlauge All i can suggest is linking your portals, not sure if it will stop anyone from finding you though

I don't want to use a Portal there, I just want to prevent someone accidentally ending up in my base through their own Portal. Since it's a bubble inside of a large lava lake it could be the nearest eligible place for a Portal to spawn if someone happens to place a nether Portal in the right spot in the over world.

Portals look for a 1x4 area of blocks with air 4 blocks above it. Anything that disrupts this should stop a portal spawning. Probably the easiest way to disrupt it is to place something in diagonal lines across the floor. Perhaps something like torches, string, buttons, grass/flowers etc. which are fairly unobtrusive and shouldn't ruin the appearance of your bubble too much.

An alternative is to build a decoy portal nearby, the game will us an existing portal if it can before generating one. When they look for a destination portal they search 128 blocks away horizontally, and the full height of the map. You could stick a portal up near the roof above your bubble and anyone using a portal that could generate one in your bubble will end up there instead.

@NutjobBob Portals look for a 1x4 area of blocks with air 4 blocks above it. Anything that disrupts this should stop a portal spawning. Probably the easiest way to disrupt it is to place something in diagonal lines across the floor. Perhaps something like torches, string, buttons, grass/flowers etc. which are fairly unobtrusive and shouldn't ruin the appearance of your bubble too much.

An alternative is to build a decoy portal nearby, the game will us an existing portal if it can before generating one. When they look for a destination portal they search 128 blocks away horizontally, and the full height of the map. You could stick a portal up near the roof above your bubble and anyone using a portal that could generate one in your bubble will end up there instead.

This is exactly the sort of information I was looking for thank you.
Since I've seen portals spawn with the bottom blocks imbedded into the ground I wasn't sure if they only needed 4 air blocks vertically to spawn, I believe you confirmed that theory.
Since my bubble only has 3 air blocks above ground level I think I'm safe. I'll keep the disruption options in mind in case I decide to add some arched cealing areas.

If you removed all valid places for portals to spawn then the game will spawn a floating one. Best plan is probably to not have portals at all.

@pand1024 If you removed all valid places for portals to spawn then the game will spawn a floating one. Best plan is probably to not have portals at all.

I don't want to use a Portal, I just want to prevent receiving one from another player.

if it cant spawn on the ground it will in the air

If you put portals ~128 blocks away from your base in the overworld any portals in the nether should link to one of those, instead of making a new one. IDK that I'd actually recommend doing that though.

be careful when blocking spawning spaces for portals because they WILL find a way to spawn in the chunk and could end up breaking bedrock above.
so i suggest creating a dummy portal (and not going through it to avoid one spawning on the overworld) and creating a trap around it so anyone who goes in it will die. (in the rare chance someone will make a portal there)