Ita-a-ak ... (© Yan Arlazorov) we take two low-frequency speakers, while one of them may be faulty (the winding, for example, burned), but both should be the same in size, ideally the same type, I took 75 GDN. You also need to have a good board with a thickness of 1.5-2 cm, the width of which is not already the size of a dynamic head with a margin of two board thicknesses, you also need a rail about 30X30 mm, shoe glue, but I think you can take a “moment”, I think it's not worth taking an epoxy, because it is not elastic (I think so, although you can try it), self-tapping screws, two screws with nuts 4-5cm long. And of course, a hacksaw, a ruler, a pencil, two straight hands, a screwdriver, a soldering iron, a drill, a pair of pliers, a low-frequency generator (I used the built-in digital tester, it gives out about 50Hz, but you can try using just a record with cool dram'n'bass) . It seems I have not forgotten anything ... although ...
We make a box with a length of about a meter and an internal size such that the dynamic head clearly enters into it, assemble it with self-tapping screws for glue.
Further, the speaker fasteners, we make four blocks from the rail and fasten them also with glue and self-tapping screws.
Now we make electrical contacts. Solder the wires (you can simply press washers) to the bolts. We drill two holes in the case (as in the figure) under these bolts and fasten them with nuts with the thread outward, and with wires inside the case.
Now it's up to the dynamic head. We take a good (working) and solder to it the recently mentioned wires. On the corresponding bolts, you can mark the polarity of the head, although this is not necessary. Next, we fasten the head to the already made fastening from the bars inside the case through rubber washers (I used plumbing gaskets) with large screws.
Then we seal the speaker in the case with, for example, plasticine (as I did), but you can come up with something better ...
We connect an amplifier to the terminals to the input of which we feed a low-frequency signal and begin to slowly move the passive dynamic head inside the case, carefully listening to the sound, try to catch the maximum volume. Missed? We draw a little for the prepared loops and again try to find the maximum.To prevent the speaker from falling in the case, temporarily attach to it the blocks of the rail with screws.
So we found the best position. Now mark this place, pull out the speaker, fasten the fasteners (bars) of the speaker also to glue and self-tapping screws, saw off the excess body. Similarly to the first, we fix this speaker, seal it with plasticine or something else (window putty, for example. You can push wool inside, as they usually do, but in my opinion the sound is getting worse.
Now, to connect to the amplifier, you need a low-pass filter (by the way, you can put it inside the case, but I have a separate amplifier for the subwoofer with a low-pass filter with a band up to 1200 Hz), you can take for example from C90 speakers (my first option).
That's all in the tweezers ... You can paint, stain, cover and cover the sides with a cloth. The amplifier can be built in, although I don’t know how to solve the cooling problem - the system is closed and not ventilated. I have not tried to finish the walls inside with soft, in my opinion it can become better.