Would love to see some of your prints on here! When I was younger my dad and I built a 3D printer, and I loved it. Since then I haven’t had the funds/time/space to get back into it but as that was like 12 years ago, I know 3D printing has gotten much higher tech.
I never designed a speaker. I have been looking for designs a few weeks ago, and there’s too much stuff out there and too little to go on if it’s actually worth it/better than HiFi speakers I already have. Maybe some day, if I find a good bookshelf speaker project that seems achievable.
Most of the kits on both sites are going to be pretty solid. You’ll be able to find reviews for most of them, as well as a bunch of forums like audio karma and what not.
As for cost/benefit of DIY speakers, DIY speakers that are well designed will punch well above their BOM cost (eg a $300 to build diy speaker will generally outperform a $300 retail speaker). But if you have a pair of decent hifi speakers already it might not be worth the dive. Most commercial speakers tune for a “house sound” so switching brands can be a bit off-putting at first. Let’s just say that burn in is a two way road and part of that road is your ear acclimating to the sound profit of the speaker.