Yea, you definately don't want to do that. 
Nope, do not build a thumbnail shadow box utilizing the >html< building block for something such as that.  The main reason being is that it's static, (fixed size realative to screen size and it doesn't dynamically update or move,,,  static), and you have to do all the media queries to fit all the different screen sizes.  To see what I'm refering to,  just check different screen sizes for yourself and you'll notice that the spacing is a little off.  That goes back to the margin spacing between thumbnails and the inside padding of the box not being big enough, or to big in some instances.  Not to mention that working with the user interface is a little cumbersome, and for me, it's just easier to write the code or piece together the snippets I need to build the block/ widget.  
  Soooo,,, in order to build a block/ widget that's recognized by the C5 framework there's a little bit of work involved.  It's allot like building modules in a Codeigniter HMVC framework.  Same general principles,,,  so if you've got that, then it should be no problem.
For a look at what's involved,,,  Here ya go; 

Blocks are the Concrete way of embedding bits of content and functionality into your pages. Learn more about how the end user adds blocks to pages here.

Out the box, Concrete5 comes with a number of blocks. However, while these are enough to get a lot of sites up and running, the first thing many Concrete5 developers will do is to play around with the blocks architecture. This might just be a change to a block's presentation layer; or you might need to create something completely new, like an event, calendar or product block. Fortunately, working with blocks in Concrete5 is a pretty simple process.

For more indepth reading and reference,,,
https://legacy-documentation.concrete5.org/tutorials/understanding-and-building-blocks-in-concrete5