Declare custom Smarty modifier in WHMCS Server Module

[nisompic]

Hello,

I am trying to define a custom modifier for Smarty templates in my provisioning module. I have tried multiple methods but they all seem to fail.
My first try was using a Hook on ClientAreaPage where my function would register a Smarty modifier:

<?php
function hr_size($bytes) {
    $size   = array('B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB');
    $factor = floor((strlen($bytes) - 1) / 3);
    $dec = 2;

    return sprintf("%.{$dec}f", $bytes / pow(1024, $factor)) . @$size[$factor];
}

add_hook('ClientAreaPage', 1, function ($vars) {
    global $smarty;
    $smarty->registerPlugin('modifier', 'humanReadable', 'hr_size');
});

Unfortunately, when using the modifier in the template file returned by module_ClientArea(), the template is not shown and I get the following error in the Activity Log: 

Syntax error in template "file:..." on line 5 "{$object.Size|humanReadable} |" unknown modifier 'humanReadable'

I have then tried this very same way by declaring global $smarty; in my module_ClientArea() function and registering the modifier there, but it did not work either.

I'm at a loss on what to try next, has anyone ever done something similar and made it work?
I'm using WHMCS 7.10 .

[Kian]

As far as I know you have to place your modifier (eg. modifier.humanReadable.php) in vendor/smarty/smarty/libs/plugins directory then you should be able to use {$somevar|humanReadable} in Smarty template. But keep in mind that last time I created a Smarty modifier was 2015 🤔

[nisompic]

I initially brushed off the idea since I thought a WHMCS update would overwrite the file, but it does work as expected, which is great!
I think I'll keep this method for now, thank you very much @Kian!