I agree. Buying an all-in-one unit where all you need to do is mount the temperature sensor and plug the heating unit into a receptacle would be much handier for most people than building the controller from parts as the OP was suggesting. The inexpensive controllers available on the internet and elsewhere are very tempting, but...
I see a lot of threads on this forum where people are asking for and often receiving advice concerning building the temperature controller from components. And often, the people giving the advice show what they've done, and it is obvious that they haven't considered and properly dealt with all of the considerations themselves. So I hate seeing people start down the road of designing and building their own temperature control system (or even trying to properly program a PID controller) if they don't have the necessary expertise to do so safely and effectively, even if it would be cheaper to do it that way.
And even with an all-in-one controller, a builder needs to be careful and do things correctly to make the final system safe and to have it work correctly. There are a lot of design considerations that most folks don't think of or deal with properly.
I see so many folks refer to a "PID" and I suspect most don't have any knowledge of or understanding of PID control systems or their proper programming and tuning. Things kind of work, and people are happy with the results, but do they really understand what's going on and how to tune the system to optimize it? In most cases, the answer is certainly no! But then again, there are a lot of very intelligent and innovative folks on this forum, and learning this stuff is not beyond them by any means.
So I greatly appreciate DIY solutions. And the DIY aspect of all of this is a big part of the fun and innovation. And I learn things every time I come on here, as we all do. And we all benefit from the experimentation of everyone on here. It's just that sometimes things shown are dangerous or less than optimum, so everyone needs to know their own level of expertise and proceed accordingly, especially when it comes to electrical work and heating systems where fire and shock hazards (among other problems) should always be considered carefully.
Heck, even the major manufacturers let things go out the door that don't work all that well or end up being dangerous!