Since every half knowledgable expert recommends the usage of broadband speakers (or indeed, one MONO) for mixing, I built a pair to the honour of the well known Auratone 5C mixing cubes. (I could optionally have bought a used pair at exorbitant prices).
The loudspeakers used in common DIY projects try to adhere to the original (original? They had about 40 models of the mixing cube ), which is a magnetically screened 5" broadband speaker. The one that is commonly used is a cheap 4" speaker distributed by an American warehouse due to availability. (It can safely be assumed that the Auratone company used the cheapest speakers that were available at the time.)
Those speakers are obsolete, because their main reason was to sit beside a CRT without disturbing its magnetic field and CRTs are mostly extinct by now anyway. So there is no reason for a magnetic screen.
This makes things much easier. Broadband (midrange) drivers are widely available. I was recommended one by a company that makes broadband speakers, namely the Celestion Truvox 0510.
The mixing cubes used to be called lovedly "horrortones". Of course, side by side with a HIFI-enclosure you miss some treble and bass, but on themselves, they sound immaculate.
Not to be compared with cheap PC speakers. Indeed, if I have my "big" speakers on (Behringer 2030) together with them, both sound so much better that you wonder how you have ever been able to live without them. But fideltiy is not the point anyway.
They don´t have any passive LRC components around them, so no phase shifts. (However they have to burn that low end energy, so don´t overdo it with loudness). No ports so no funny bass response.
From my past enclosure builds I had 3/4 inch birch plywood over, which may be a bit over the top but does not harm. The edges are bevelled with a router (some of the mixing cubes had that, some not), the speaker is front mounted with a recess milled.
Laquer is Liquitex Professional spray paint for great colors with a clear glossy cover.
Note the 5c´s logo with is my hommage to the Auratone.
The interior is stuffed loosely with some Basotect foam left over from the last room treatment.
Screw-on terminals on the back make the connection to the outside world.
I use an elderly HIFI amplifier to drive them, which in turn is fed by one of the outputs of my sound module. Its mixer allows to turn on the speaker sets at will and MONO them.
Total cost including drivers, wood, spray paint, p&p is 85 Euros for a pair.