How to pass checkbox value to echo $pod->filters()

Hello! I’ve been looking for a solution for the past few days but I just couldn’t figure it out. Under echo $pod->filters(); , I echoed some checkboxes from another pod page which will let visitors search my pods through any custom fields they want:

<form action="" method="get">
<input type="checkbox" name="bookauthor" checked="checked">
	<label for="bookauthor">Author</label>
<input type="checkbox" name="bookyear">
    <label for="bookyear">Year</label></form>

Then I set my 'where' variables in php.

$bookauthorfield = array(
             'key' => 'book_author',
             'value' => pods_var( 'search' ),
             'compare' => 'LIKE'
          );
$bookyearfield = array(
             'key' => 'book_year',
             'value' => pods_var( 'search' ),
             'compare' => 'LIKE'
          );

After this, I used if, else, and isset in an attempt to get the checkbox value and pass it to my $params. If the checkbox is checked, it will search for the field. If not, then it will do nothing.

if (isset($_GET['bookauthor']))
	{ $searchbookauthor = $bookauthorfield; }
else
    { $searchbookauthor = NULL; };

if (isset($_GET['bookyear']))
	{ $searchbookyear = $bookyearfield; }
else
    { $searchbookyear = NULL; };

Then I changed my $params into

$params = array(
    'orderby' => 'book_year DESC',
    'limit' => 5,
    'search' => false,
    'where' => array(
         'relation' => 'OR',
          $searchbookauthor,
          $searchbookyear
	)
);

But sadly, that didn’t work nor the dozen attempts I made during the past few days. =(
How do I connect these checkboxes to echo $pod->filters() ?

https://www.ads-software.com/plugins/pods/