If our OP is really into DIY and wants to learn something, I'd like to offer a different perspective on that: building a plan/kit of someone else's design does not teach them how to analyze the parts they have on hand (which is a foundational skill of speaker design), or how to make the engineering choices needed to design a good speaker, like how to weigh tradeoffs, or how to select a crossover frequency and slope.
Yes, failure can happen. But it's very unlikely to be absolute failure, and something can always be learned from it. And knowing that failure can happen can motivate a person to make economical choices.
As far as the cabinets go, although I don't disagree that a cabinet can adversely affect a speaker's performance, but I'm not of the opinion that the OP's cabinets are unredeemable. And finding out their level of usability is part of the process of learning how to make design choices and weighing the tradeoffs.
But that's all if the OP is really into DIY or wants to learn something. If they just want a nice pair of speakers or to just build something, I completely agree that the used market or a kit is a much better choice.
I speak from 40 years of experience in loudspeaker development, both in the professional DIY sector and with other developers, manufacturers, and driver manufacturers.
I've also overseen DIY projects on numerous occasions.
And even though today's technology makes loudspeaker development easier, projects like the one the thread starter describes almost always go wrong.
I don't mean that you can't listen to music with such a loudspeaker, but rather that these speakers would be blown out of the water by, say, a pair of Elac DBR62s. And those only cost €400.
It can even happen that the result is worse than the original product.
And that's precisely what I wanted to warn him about and offer some food for thought, especially regarding these enclosures, because I know how much of a difference they make. Above all, ordinary users are completely unaware of the inferior materials used to make these enclosures, which cost less than €20 to manufacture (not just the materials, but the entire production process). The material is usually not MDF, which also comes in very low-grade versions, but rather the cheapest and thinnest particleboard, which has less than a quarter or a fifth of the strength of an 18 or 19mm MDF board. The cheapest IKEA furniture is of much better quality.
And regarding your first paragraph, I completely disagree.
There are thousands of DIY speaker plans and kits available worldwide. How could he, especially at the beginning, learn more effectively and quickly than by looking at, studying, and analyzing them? He can see how the enclosures are constructed and dimensioned, which drivers were chosen, how the frequency responses match up, how different crossovers are designed and how they can lead to different results even with the same drivers, assembly instructions and notes on what didn't work, experience reports, and speaker descriptions from developers and builders. And much more besides.
In the past, I've learned at least as much from the mistakes and successes of others in such projects as from my own experiences, and that's still true.
And one of the most important, and by far one of the most difficult, aspects is selecting compatible drivers. The potential result hinges on this, long before anything else. With an existing enclosure and perhaps even pre-existing cutouts, this becomes many times more difficult.
If
@unmutual is aware of all this and wants to undertake the project, then that's his decision, of course.
But I think it's important to consider all sides, including the less appealing ones, because there are reports in this forum, as well as in others, from people who have spectacularly failed even with simpler DIY speaker projects.
And no matter how easy it has become to take measurements, only a fraction of the speakers on the market are good, and a much smaller fraction are outstanding. By far the largest fraction is... Mediocre to terrible. Seems to be not so easy, even for manufacturers.
