Create a custom block with a form using ECK

custom_module.module

<?php
include_once('ProdAvailabilityRepository.php');

function custom_module_block_info() {
  $blocks['product_availability'] = array(
    // info: The name of the block.
    'info' => t('Product availability form'),
  );

  return $blocks;
}

function custom_module_block_view($delta = '') {
  // The $delta parameter tells us which block is being requested.
  switch ($delta) {
    case 'product_availability':
        // Create your block content here
        $block['subject'] = t('Title of first block (example_configurable_text)');
        
        // get repository object
        $prodAvaRepository = new ProductAvailabilityRepository();

        // getItemIdFromProdAndRelay, get an id
        if ($avai = $prodAvaRepository->getItemIdFromProdAndRelay(arg(1), arg(3))) { // if entity exist, edit mode
            // load the entity using the ID
            $entity = entity_load_single('product_availability', $avai);
            $form_id = "eck__entity__form_edit_product_availability_product_availability";
        } else { // if entity doesn't exist, add mode
            $options = array('type' => 'product_availability');
            // create an empty entity
            $entity = entity_create('product_availability', $options);
            $form_id = "eck__entity__form_add_product_availability_product_availability";
        }
        
        // create form_state needed by form
        $form_state = array(
            'want form' => TRUE,
            'build_info' => array(
              'args' => array($entity)
            )
        );

        // Use module_load_include so that caches and stuff can know to load this.
        form_load_include($form_state, 'inc', 'eck', 'eck.entity');
        $form = drupal_build_form($form_id, $form_state);

        $block['content'] = $form;

      break;
  }

  return $block;
}

// ALTER FORM
function custom_module_form_eck__entity__form_add_product_availability_product_availability_alter($form, $form_state)
{
    custom_module_form_eck__entity__form_product_availability_product_availability_alter($form, $form_state);
}

function custom_module_form_eck__entity__form_edit_product_availability_product_availability_alter(&$form, &$form_state)
{
    custom_module_form_eck__entity__form_product_availability_product_availability_alter($form, $form_state);
}

function custom_module_form_eck__entity__form_product_availability_product_availability_alter(&$form, &$form_state)
{
    if (path_is_admin(current_path())) {
        return;
    }
    // your content, like hidding entity reference field
}

ProdAvailabilityRepository.php

<?php 

class ProductAvailabilityRepository {
    static function getItemIdFromProdAndRelay($productId, $relayId)
    {
        $dbQuery = db_query("
            SELECT eck_product_availability.id AS id
            FROM {eck_product_availability} eck_product_availability
                LEFT JOIN {field_data_field_relay_ref} field_data_field_relay_ref 
                    ON eck_product_availability.id = field_data_field_relay_ref.entity_id AND field_data_field_relay_ref.entity_type = 'product_availability'
                LEFT JOIN {field_data_field_product_ref} field_data_field_product_ref 
                    ON eck_product_availability.id = field_data_field_product_ref.entity_id AND field_data_field_product_ref.entity_type = 'product_availability'
            WHERE 
                (( (field_data_field_relay_ref.field_relay_ref_target_id = :relay_id ) 
                AND (field_data_field_product_ref.field_product_ref_target_id = :product_id ) ))
            LIMIT 1
            ", array(
                ':relay_id' => $relayId,
                ':product_id' => $productId
            ));
            
        $results = $dbQuery->fetchCol();
        return array_shift($results);
    }
}