You will see trays advertised that are similar to stock mouth
protectors--you heat in hot water and form to your teeth with your
fingers.  Obviously they don't fit nearly as well as vacuum-formed
custom trays made by a dentist.  Here the risk is primarily burning of
the gums from the bleach that leaks out.  Most of these kits therefore
sell a less concentrated bleach to decrease this risk.  However, I have
seen advertised kits like this that claim to have 15 or even 20%
carbamyl peroxide.  I think using bleaches this concentrated in stock
trays is risky; I wouldn't do it.

Steve