I've been thinking about this idea for so long and have never really settled on the right design, but to heck with it, this is version 1.0!
Step 1: Stuff You Need
The trouble with this whole idea is that every tub seems to be a little different, so this is pretty much a one off based on the design of my bathtub. I would have considered using suction cups, but they don't stick to the textured surface of my tub, so that won't work.
desk surface: Some sort of sturdy material, preferably sheet plastic of some kind.
two shelf braces.
nuts and bolts.
waterproofing chemicals (Varathane or something like that)
pin vise, drill
coping saw or similar
Step 2: Measure, Cut
To cut the fiberboard I used repeated careful strokes along the edge of a t-square until I finally made my way through the material. This took about 30 repeated passes, but I wanted a clean cut.
Step 3: Assemble
Just attach the shelf brackets to the back of the board with the bolt and nut.
Step 4: Add the Hook
My tub has a handle along the side, so I'm attaching this modified clothes hangar to hook onto that and give it some stability.
I bent the hook a bit to give it just the right angle to hook securely.
Step 5: Attach Hook
Make sure inspector kitty approves.
Step 6: Waterproof
I gave the thing two complete coats (with 1 hour drying time between them) and used half the can. It soaked in pretty readily - I just don't feel confident that it will resist water. This stuff stinks too. I'm hoping that letting it air out overnight on the balcony will resolve that. We'll see.
Step 7: Tub Desk!
I picked up some corner molding to be the pencil tray at Home Depot. It is the most expensive part of this project. the molding cost $5.40 plus the gas to drive over there made this part of it kind of pricey. Oh well.
Obvious improvements that could be added: adjustable height, place to hang a little towel to dry your hands for turning pages, some sort of c-clamp attachment for the side of the tub for better stability.
Final word of warning, don't put anything on top of it that you couldn't stand to see dumped into the tub. Especially don't put your laptop or other electronic devices there, too risky. Proceed at your own speed with due caution.
Hope you enjoy this idea and I welcome suggestions for improvements.