My wife needed a way to organize her sewing thread spools sew they wouldn't be in a giant pile. Sew I designed one that would fit the dimensions of the drawers she was storing them in and gave it a handle sew she could easily take the whole thing out whenever she needed to. I've got two sizes, one that's six spools deep and another that's seven. 

I designed this with my Prusa XL in mind but the smaller version should probably fit on an Mk3, Mk4, or similar, as it's 245 mm long. 

Also, supports are required just for the handle and I was originally removing them but I actually think they give the handle a little more sturdiness, so now I'm just leaving them on. 

Make sure you have enough perimeter walls to give it the strength it needs. I'm using a .6mm nozzle and I use 4 perimeters. I also use 5 solid bottom layers and 4 on top. With those settings, I'm getting good results with 20% infil printing with PLA.