Witam. Może po prostu dać border dla iframe.
<div class="container">
<iframe class="iframe-border" width="100%" height="300" src="https://www.youtube.com/embed/XGSy3_Czz8k">
</iframe>
</div>
body{
background-color: grey;
}
.container{
position: absolute;
width: 300px;
top: 50%;
left: 50%;
margin-right: 50%;
transform: translate(-50%, -50%);
}
.iframe-border{
border:20px solid black;
border-top-left-radius: 20px;
border-top-right-radius: 20px;
}