DIY

Kits are a good way to start, I would recommend the Hitmaker MT, it is more neutral than most and gives you a good reference point for future builds.

https://www.parts-express.com/Hitmaker-MT-Bookshelf-Speaker-Kit-with-Knock-Down-Cabinet-300-7116?quantity=1

The kit comes with a cabinet so you just need to glue it together.

.25" is too thin, you would need substantial bracing to avoid audible resonance. If you do laser cut a speaker cabinet you will want to bond several layers together.

I would also recommend purchasing a measurement mic. Being able to apply EQ based on your actual in-room frequency response is invaluable.

https://www.minidsp.com/products/acoustic-measurement/umik-1

If you don't already have a pair of good subwoofers, that would be a strong addition to your setup. Bookshelf speakers cannot really play the bottom two octaves of music properly, and even if they could it isn't ideal. Room modes limit bass performance, optimal placement for bass quality usually conflicts with optimal positioning for stereo imaging.

For DIY subs, an Ultimax 15" in a 4ft sealed enclosure with a Crown XLS2002 is good value.

https://www.parts-express.com/Knock-Down-MDF-4-cu.-ft.-Subwoofer-Cabinet-with-Blank-Baffle-300-7088?quantity=1

https://www.parts-express.com/Dayton-Audio-UM12-22-12-Ultimax-DVC-Subwoofer-2-ohms-Per-Co-295-512?quantity=1

https://www.crutchfield.com/S-iOpiNYr307W/p_859XLS2002/Crown-XLS-2002.html

The XLS2002 may seem expensive, but it has enough power to drive a pair of subs. If you own a table saw then you can make the cabinets cheaper. Either way you will need a jigsaw or router to cut the driver hole.