This boot fits the 40oz Hydroflask cup with silicone straw (as seen at Costco). It is intended to be printed out of TPU. The model is parametric so feel free to tweak it to meet your specific needs. Here are the attributes that can be adjusted:

For mine I used Sainsmart Teal TPU with the default Sainsmart TPU profile in PrusaSlicer. I printed it at 0.15mm layer height with 15% gyroid infill.