Ratchet straps or... Wow the name just totally escaped me... Oh! Bungee cords.

ETA: also planning on putting these furniture pad things on the crib feet to further reduce movement https://www.amazon.com/gp/aw/d/B06XPW77TH

https://www.amazon.com/dp/B06XPW77TH/ref=cm_sw_r_cp_apa_glt_fabc_P23E07D2M1Y0VFCEES32?_encoding=UTF8&psc=1