I struggle with the boundaries as well. Especially at the end of the day or before lunch. "This will only take me 1 minute" fixes turn into half-hours and ruin the whole schedule.
One thing that might be worth trying is to just stop working whenever your time chunk is done, regardless of what's left. I remember reading an article here on how Hemingway used to stop writing in the middle of a sentence at the end of a creative writing session. The author was suggesting that programmers do the same thing (leaving a half completed function or even line of code, even if it means that the project isn't building).
I tried it after reading the article and I was very surprised with the results. I find it much easier to start in the morning when I have a half completed line of code in front of me. The context comes back almost instantaneously and even when it only takes me a minute to finish what I had started the day before, I feel like I am already in the flow and I will be much quicker when I start the next task. When the first task on my list is to start a feature for example, I often end up browsing the code files aimlessly trying to find the starting point, etc.
Ever since I started using this technique, I find it much easier to just stop at the end of an allocated time chunk. It's only a question a perception obviously, but instead of feeling like I have given up on a task before it was done, I think of it as a way to set myself up for the next time chunk.