Too much thought went into this:

After 6 months of daily use, the soap tray in the bathroom took a flying death leap. It fulfilled the design criteria of preventing mushy soap, however I expect better durability from a soap tray, especially one designed by OXO. (the suction cup split from the tray part)

http://www.organizeit.com/pooxosoapdish.asp

Normally this would not be news, however being cheap and incredibly nerdy I have not yet bought another one due to

1) Waiting on one of the Bed Bath Beyond 20% off one item coupons (since arrived)
and
2) Wondering what design criteria would make a better soap dish, and if such a thing exists.

This is where I make rather odd use of the internet.

I wonder how many daily Google searches there are for 'best soap tray design'

According to the results there are 170,000 related pages. However none seem to be related to either 1) making your own or 2) soap tray functionality and durability reviews.

It appears the weak point of suction cup designs are the cups themselves, so those are out.

Hanging from shower head devices are promising, though bulky and generally more expensive than trays.

Trays are cheap and generally indestructible, but edge-of-tub space is limited.

I know it really doesn't matter, but sometimes the engineering mind takes over.