Sandals? Maybe, but I have doubts. Cycling sandals may help but it will likely be not because they provide ventilation but because they are stiff. Why should this numbness happen only five miles into a ride? Foot numbness is more likely to be caused by nerve pressure than by heat and if you are getting this so early in a ride you should take it seriously. I'm guessing stiffer soles and possibly an arch support are what you need. Also, make sure you are not mashing too tall a gear and putting unnecessary pressure on the ball of your foot.

To my knowledge there is no insert or insole that will provide the stiffness you probably need. That has to come from the shoe and it still may not solve the problem if the arch support in the shoe isn't sufficient.

Do a BF search on "neuroma" and you will find interesting reading. Here is one of the better posts on it:
http://www.bikeforums.net/showthread...hlight=neuroma