Using Tinkercad I started out designing the model as a solid block and increased the edge radius to make is smooth and comfortable to grasp without needing to sand it down.
Next, I added a tall cylinder with a 5mm hole to the middle of the block. The hole was made big enough to accommodate the size of most any pull string.
I printed it on a Monoprice Maker Select v2 with Cura Coarse settings 30% infil. I used Monoprice PLA.
Once the string was threaded through, you just tie a knot to keep the handle from coming undone.