I guess I should explain my trap then. You've almost got it right Myroc. Firstly as my video shows the hallway is clear of all water when the invaders are charging up it. I release the water from a small reservoir to the west. I'm not certain but I think it's all water bellow or equal to 3/7 that turns to floors rather than icy walls, I had to make the reservoir bigger to make sure the whole lot froze. One other odd quirk I noticed is that magma that is standing still doesn't seem to stop water freezing, admittedly though this might be because I used tweak + for each tile to get some magma, I got bored looking for a site with both ice and magma.
I'm going to spoiler all the images but I'll explain in detail how my trap works, in hindsight there are things about it which could be more efficient.
On my map I dug down to the rock to build my machine in so I'll leave out pictures of the top few z-levels as its just ramps. Here is the first working floor. The big room on the left is the evaporation chamber, it deals with the excess water generated by the trap. On the right there are pumps that pump water out of the killing floor through fortifications. You'll notice there is an ice floor/roof over the killing floor, this is what is left behind when melting unsmoothed ice walls with magma.
The smooth ice floor is the killing floor where all the freezing takes place. On the left you can see the water reservoir and the floodgates that hold it's water back until necessary. There are fortifications to protect the floodgates and another load of fortifications on the right which the pumps above pump water through. They might seem unnecessary but I cant let the water freeze under the pumps without it breaking the system. On the far right there is some plumbing, the pumps above pump water down through these tiles.
The magma defrosting room. On the far left there is some plumbing which feeds the water reservoir above from bellow and on the far right some plumbing which is fed from above. In the middle you have mainly open space with two bridges and some pumps, when turned on it allows the magma to cycle through this chamber heating the one above. I used bridges because I can drop all the 1/7 magma into the chamber bellow when I'm done using it which saves on magma through stopping it evaporate.
Main magma chamber. This room is two z-levels deep as I thought 1/7 magma wouldn't evaporate when on top of 7/7 magma but was wrong. I'll only post the one z-level of this to save on space. Once again there is the plumbing on both sides of the main chamber, on the left pressure forces it up and on the right pressure forces it down.
Where the plumbing finally meets.
The most complex part is probably the water plumbing. I made it far too complex and if I ever did this again I wouldn't dig down and feed it under the magma chamber like I have. Here is a simple image explaining the water plumbing.
Warring this next image is a huge but it is my refined freeze trap blueprints.