I also have plans for a great DIY microphone for only $0.50.  Materials include two paper cups and a length of string.  It works really well because I said so on the internet.  
 
Ah sarcasm...will you ever get old?

I'd think you'd get more inspiration singing into a waste basket.  Could probably wear it too, with those headset mics you see preachers using or air traffic controllers.

Years ago when I was struggling to figure out what to do with my vocal space, I needed to convince myself that the problem really was comb filtering from windows and walls too close by. (As opposed to the need to buy a more expensive microphone!)
 
As an experiment, I pulled my heaviest winter overcoat from the closet and threw it over both my head and the microphone and recorded a test. It was awkward but I was amazed at how effectively the yuckiest components of the sound vanished. That was all the convincing I needed, and I was able to confidently commit to a strategy of 360-degree absorption that did indeed prove to be the solution. (Emphasis on "360".)
 
There are several problems with the foam-lined box approach. The first is that you're applying absorption in the wrong place - behind the microphone, in a cardioid microphone's dead zone. Second, the type, thickness and density of the foam or batting is inadequate to absorb any of the most significant frequencies. Third, there will be new resonances within the box itself that exacerbate the problem even more. The box will also not have enough mass to avoid vibrations conducted through it, whether laid upon a desk or mounted to a stand.
 
In order to be effective, you have to absorb as much reflected sound as possible, from all directions. That means placing absorption behind the singer, not behind the microphone. Even more significant is the ceiling, which will be the closest boundary in most rooms for a standing singer. If the singer is 6 feet tall and the ceiling is 8 feet high, there will be a resonance at the 2-foot wavelength - about 500 Hz, smack in the middle of the most-significant harmonics for a male vocalist. And, of course, repeated at every 500 Hz interval all the way up the audio spectrum.
 
A makeshift vocal booth is still do-able on the cheap. Perhaps not for $23, but certainly for a couple hundred dollars. Use at least 3 inches of rigid fiberglass (4" is better) and set it up as far as possible from walls and windows. Put extra thickness between you and the nearest wall. You can do this using free-standing panels, so it's possible even for those living in rented apartments.
 
 

You said it much better than I could have Bit. I was trying to make the point that it's what is bouncing from behind that matters more with a cardoid. I think the idea behind those front mic stand mounted diffusers is that you are stopping what would have bounced off the wall in front of you and wound up in your mic after it bounced off the wall behind you.
 
Those diffusers look like a simple frame with some foam on it but the workings are a little more complicated and trying to make a home made one is kind of hit and miss unless you know some of the physics behind the sounds you're making and then can taylor it to that.Probably just as important like you say...is what is immediately behind the mic besides the singer.

I used to think about that kind of stuff too.... the shape of the room, the wall coverings, the floor & ceiling, reflections and all that technical stuff.
 
Not anymore. I have a non-treated room and my wife won't let me convert it to a "proper studio" so I'm stuck.
 
What I have discovered is that as I have learned a few things, the vocal and guitar recordings I am now getting are vastly superior to the ones I did several years back and nothing other than what I know and how I apply it has changed. No blankets and no absorbers, just me trying to get the best vocal and acoustic guitar sound I can muster under the circumstances.
 
I can still hear the cars, trucks, trains, and dogs, and I have to stop or punch where that's really evident in the track.  But I think I'm gaining some headway. I wonder what I could do if I had a treated room in a quiet area of town.
 
All that to say, I think if you apply yourself and have some halfway decent gear and use it correctly, it really won't matter much whether you have a treated room and absorbers and such.

New kind of sound - using the inside of a refrigerator, but first I have to eat all the food and then take a dum..........!
 

That will only work if the fridge is stocked with vegetables. I hate to think what the acoustics would be like with nothing but beer cans in there.