I try to extend the NumericField class to create my own fields in a model admin. Here is the code I use :

class EuroField extends NumericField
{

    public function __construct($name, $title = null, $value = '', $maxLength = null, $form = null) {
        parent::__construct($name, $title, $value);
        $this->setFieldHolderTemplate("EuroField_holder");
    }


    /**
     * {@inheritdoc}
     */
    public function Type() {
        return 'euro text';
    }
}

And now the template file : /themes/mytheme/templates/Includes/EuroField_holder.ss :

<div id="$HolderID" class="field<% if $extraClass %> $extraClass<% end_if %>">
    <% if $Title %><label class="left" for="$ID">$Title</label><% end_if %>
    <div class="middleColumn">
        $Field <span>THIS IS A TEST</span>
    </div>
    <% if $RightTitle %><label class="right" for="$ID">$RightTitle</label><% end_if %>
    <% if $Message %><span class="message $MessageType">$Message</span><% end_if %>
    <% if $Description %><span class="description">$Description</span><% end_if %>
</div>

Do you know why the template is not used?