I've been thinking about this especially for Oklahoma onion burgers:

Dexter Outdoors 4" x 5" Hamburger Trowel https://www.amazon.com/dp/B006WQKQP0/ref=cm_sw_r_apan_i_P6QZ2K9QQQD8E39YE67D?_encoding=UTF8&psc=1

Downside is I think it's carbon steel so you'd have to watch out for rust.