It is possible to build speakers. I have done so for many years. In fact, I have had dozens of different speaker systems and built almost all of them by myself. I started out by putting a 12 inch full range speaker into a 4 cubic foot sealed box. It sounded pretty good, and impressed my high school classmates. How deep you go will depend on what you want to do. You can figure box volumes, and that is easy, but you need to decide what you want to build. With 30 watts RMS you will need either to listed close to the speakers, or you will need to build efficient speakers.
I looked at the website suggested by thermisto462. Kits are a good idea if you have no experience, and you want to build something. You can build the kit and just assemble it from precut wood and speakers and other componenets, but looking at the site suggested by thermisto462, you may find that those kits are too expensive.
I will suggest a kit from Parts Express, http://www.parts-express.com
, a reliable U. S. merchant that will come close to your price range. It is the Dayton Audio BR-1 kit. If you just buy the speaker components, you will have a pretty good speaker system for close range listening, even though it is not efficient. The components sell for $134.00 U. S. or about 152 Euros. It's about as cheap as you can go for a good 2-way system.
You can also buy a full range speaker from Fostex and build the box, using the Theile-Small parameters to calculate the volume.
As another suggestion, here is the very 12 inch full range speaker that I used for my first good full range system: https://www.ebay.com/itm/Mustang-M-12D- ... Sw8axZ8WZ9
Put 2 of these in a pair of 4 cubic foot closed boxes, and it will sound great, though not "audiophile," and it will be really efficient, giving good scale even at a distance, and even with a 30 watt amp. take up a lot of space, though.
Best I can do for now. And good luck from that wood- butchering old speaker builder (since 1963),