Reddit mentions of 10pcs Ladder Lock Slider Plastic Buckles Belt buckle Package Accessories Black (1"(25mm))

Either cut the long strap and re-sew it at a more adjustable length, or replace that buckle with a ladder lock style, which will let the excess hang out the back. Something like this, but sized appropriate to your strap width: https://www.amazon.com/dp/B0734X91FR/ref=cm_sw_r_cp_apa_02CZBbRG6MK6E

Edit: or just fold it over and use two safety pins parallel to the fold (vertical) to lock it in place. Poke in and poke out on the same side so it's not rubbing against your head.