Hey Mark, Im not sure if this will help you out or not, but this is a screen cap from UBBDev's current YouTube Custom Tag for UBB.threads 7.6.0 (Snapshot 20160813)
There is also a "common.css" file which is unique to UBBT v7.6.x series.
You can paste the following code in to your style sheet's EXTRAS section, if you are not currently on a 7.6.x series install. You'll need to do it to each of your styles. And then when you move to 7.6.x, you'll probably need to remove it at that time, since it will duplicate functionality.
/* video embeds */
.video-container {height:0;overflow:hidden;position:relative;padding-bottom:56.25%;padding-top:30px;}
.video-container iframe,.video-container object,.video-container embed {height:100%;left:0;position:absolute;top:0;width:100%;}
