The inserts for IKEA SAMLA crates tend to bend and then slip down. This part helps stabilizing them. It comes in two sizes for the discontinued inserts for the large crates and the still available inserts for the mid sized crates. Print two of them and insert them into the gap between the walls and the handle part of the inserts.
I printed them in PLA with 1.2mm wall thickness, 15% infill, and 0.3mm layer height.
As the crates are a little bit slippery and wedge formed you might want to glue them down.