[WIP] Overture
I downloaded the map from here and I didn't get stuck anywhere in this map.
I see the map is basically
Hang on, you kept the first BOX until the fling (Chamber 02)? Which box, exactly? I'm kinda hoping that was a typo, otherwise that would really be something.... 
Nope, the first one. You can lean it against the door and take it with you, since the fizzler doesn't work. Just lean it on the door, hit the button, the door opens, then you run through and crouch and walk through, and wait for the box to follow. In the next room you now have two boxes, so just put one on the button and carry the other with you to the elevator, since the fizzler doesn't work. Then you just solve the portal puzzle in the next room and take a box with you, then the fling chamber, right? It's too hard to keep a box with you or throw it from the last fling into the ground portal, but even if you could get a box to the fizzler (use noclip) that one destroys it.
I like keeping boxes as long as I can.
Just played it, the puzzles are fantastic. I have read the readme, and I have another bug for you 
and that is, all your lights are white, but portal lights _should_ be blue-ish (I don't know the rgb combination by heart but you can find it somewhere, probably in the wiki). Really, this killed the atmosphere 
(percentage calculation sphere self-test failed!)
108.1% done, -8.1% remaining...
Released here!
All maps for Portal (1/2) worth playing are mirrored here.
and that is, all your lights are white, but portal lights _should_ be blue-ish (I don't know the rgb combination by heart but you can find it somewhere, probably in the wiki). Really, this killed the atmosphere If you want to have the water change then make 2 func_brushes.
One of these has the posinous texture and the other has a normal water texture, the poisonious one should be enabled and the normal should be disabled. Now, once the chemicals have been put into the posionous liquid have it so that the posionous one become disabled and the normal water one becomes enabled.
And thats should be your interchanging water.
Edit: Now here's a lesson that every mapper should learn before they start mapping.
When you create a light source go into its properties and set the constant option to "200,000" this will make it so that areas are neither overbright or underbright.
Now, for something more portal related:
The standard colour for all portal chambers is "183 204 218"
For BTS areas, buttons and box dropper parts the colour used is "251 159 57"
Now, if you ever forget this, i will murder you.The amount of people i see don't do this is really, really annoying
Nice Portal 1 map, it was really great to play some 0p levels with mounted autoportals. there were a lot of puzzles that could have reused its own elements more... like taking a cube with your through a fling, however that is probably something more done in portal 2 maps, because of how easy it is to hold onto cubes.