multiple dropdown custom fields not checkbox

I am wondering if there is a way to modify the dropdown selector to be able to select multiple search custom field. I know that I could use checkboxes for multiple terms but I have a huge list of cfields and I don’t want all of them to display unless the user clicks on the dropdown item. More than likely the user will need to select more than 1 fields so I am hoping that by holding (shift) or (ctrl) that they can select/highlight more than 1 fiield at a time.

I have tried to add multiple=”MULTIPLE” within the code of the uwpqsf-front-class.php file but it doesn’t seem to effect the front end form.

Any help is much appreciated!!