I can’t believe I’ve been balancing models on their roofs for my entire life. Several manufacturers offer foam cradles, but I’ve always felt it was something I could make myself. Then I didn’t. For years, no decades!
Well, no more. And since it has taken me so many years to finally make my own cradle, I over-engineered the hell out of it so I won’t have to put off making another one. This one is 3/4″ and 1/4″ plywood, and as it’s the last one I will own*, I painted it white. With my habitual two layers of primer and two layers of paint, this luxury accounted for the majority of the construction time.
My models tend to be delicate, and I don’t like the idea of trying to slide them into a foam U, no matter how soft the foam is. So, I hinged one side of the cradle and latched it with a magnet. The model sits in the remaining L, and then with the last side folded up, it is snug and secure.
* The last one I will own unless I change scales.
Pembroke:87 
I like the front door idea. I hate trying to squeeze my models out of the foam channel, wondering what piece will catch on the foam and rip out.
Exactly! If you do use a simple channel, however, you might be able to improve performance by wrapping the model in a piece of plastic bag so details don’t catch.
I keep meaning to build one myself. And haven’t, for decades. Maybe it’s time I address that too.
I was thinking about a simple sling rather than foam. I do like you magnet idea. Any chance the frog will accidentally pop open while working on a model?
Oh, a sling is an interesting idea! Would it hold the model securely, or would it rock in the sling? I guess there’s only one way to find out.
Now that I’ve used it, I’m really liking the solidity of the model in the cradle. The magnet is pretty weak, but easily withstands the forces I put on my models so far. Typically by the time they’re upside down, models are pretty far along, and not subject to much hammering or prying.