This is a remix of the master seat By rawnei done with my poor modeling skills in blender and tinkercad combined. It is meant to fit in the handle for the trolley pretty snugly so you really shouldn't need anything else to secure it down, it does make it so you have to take it off to use the trolley but I don't mind it. Basically follow the same directions as Rawnei's Master seat with the infill mesh of a different density to get more squish on top. I used 10% infill and 6% on top with gyroid infill on mine and it might be slightly too squishy. With harder TPU you may want to lower it to 8% and 5%. Also the model didnt quite come out how I wanted to so orient it the way I have it but you gotta sink it about a mm below the buildplate to get a nice flat first layer. ( I am not a 3d renderer). Other than that some tree supports for the insert part and follow Rawnei's guidelines for the fuzzy setting in cura, really makes it nice.