Fortunately, although they don’t mention or link to it anywhere, you can still go directly to the old URL to create an modify these custom players /cp/.

Here is the link to create one:
http://www.youtube.com/custom_player

Here’s a link to view / edit them:
http://www.youtube.com/my_players

And here’s what such a player will look like after you’ve done it:

Whereas the new player looks like this:

PS: Unfortunately, there appears to be no way to resize the video to be contained within the boundaries of this content section. And ironically, the new playlist embed code will. So, pick your poison!

PPS: Note that this old method also appears to circumvent a “embed prevented by request”, as otherwise I’d offer the same set of videos with the newer player.