FULL CREDIT GOES TO NightfallVT ON THINGIVERSE.

The normal size has a 160mm internal (max roll size) and 32mm post diameter (max hole size). The medium and small sizes are 140mm and 114mm respectively. The jumbo version has a 180mm internal and 38mm post diameter to accommodate larger rolls.

After adding the towel roll with the end free, spinning the roll clockwise should automatically feed the end out of the side slot. Tear a sheet off by pulling from the bottom upward and towards the right edge of the slot.

Feel free to increase the holder's height. I chose 180mm high to fit my bed, and I don't think I would go much shorted, but I think taller wouldn't hurt.