Difficulty question
I would recommend hving a panel that blocks the wall whenever you walk near a wall, and have the puzzle based around that.
so, lets say you grab the cube after placing the 2 portals. you walk near the entry, where you toss the cube onto the button. if you try to actually walk through or very close to the wall, panels will burst out of the ground to block you, and then retract once you are a ways away.

Specially with one portal people are more limited to what they can do. 1p can be great to learn new concepts
lpfreaky90 wrote:
I think you'll be fine with that
Specially with one portal people are more limited to what they can do. 1p can be great to learn new concepts
so true. it forces you to think even harder, even if it does sometimes give away the answer.
If you need help creating it: make sure you actually release content (or showcase it) and show progress before asking. There are so many examples of people being the 'ideas person' and the project then quickly dies.
Remember that a mod can take months on end (perhaps maybe a year or more) to create something so large and of good quality. So be patient 
on topic: I think many people would enjoy a difficult map pack. I highly enjoyed portal prelude and its' puzzles.
I enjoyed Portal Prelude too but those last few testchambers were really too hard. I'm thinking mostly of the second part of Chamber 19.
wrathofmobius wrote:
So my main question is this: in my third chamber, I have a button that the player must place a cube on through a portal, but without going through. Is this too much to ask for the third chamber?
You could make a small puzzle leading up to this, to teach players the technique, like Valve did in Portal 2 with the flinging. (The part where you have to stop a flying box by flipping up a panel in front of it.)
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First, put a button high up a wall on a single 128x128 platform, where the only way to reach it is by putting a portal on the wall right next to it. People will instinctively stick the box through first and then step back or turn around, without actually setting foot on the platform.
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Then repeat this, but stick some barrier in front of the portal which allows the box through but blocks the player. (I'd suggest a window with a square, box-sized hole in it.) Players will remember what they did earlier, and realise that this will still work and that they don't actually need to walk out onto the platform to deliver the cargo.
wrathofmobius wrote:
I enjoyed Portal Prelude too but those last few testchambers were really too hard. I'm thinking mostly of the second part of Chamber 19.
Totally agree. Especially the GLaDOS battle was Nintendo hard. I found everything before chamber 19 pretty enjoyable. And I thought it was a clever idea that they made it so that you could leave #18 without the cube, but if you did, you'd have to come back for it later.
Good luck with your mod. It sounds promising!