I use the simple amazon basics ones. The medium size fills about 3/4 of the internal area (leaving room at the top for toiletries). There are also "long" cube or skinny ones. One of those fits along the side of the medium cube. Or you could do 2-3 small cubes depending on how full you pack them.

https://www.amazon.com/AmazonBasics-4-Piece-Packing-Cube-Set/dp/B014VBIS6G/ref=sxin_9_pb?cv_ct_cx=packing+cubes&dchild=1&keywords=packing+cubes&pd_rd_i=B014VBIS6G&pd_rd_r=b8743919-d7cd-460c-a4c4-0c2087f5758f&pd_rd_w=8s4q0&pd_rd_wg=ze7Wd&pf_rd_p=3e19f259-22e6-4c08-b8dc-02f70ea1fe7d&pf_rd_r=5BHK3G4RKW5K3864B1EV&psc=1&qid=1599048591&sr=1-2-8065ff8c-2587-4a7f-b8da-1df8b2563c11