Hi!
For starters, I'd leave out the whole wheat flour and corn starch.
These two ingredients will only make the cake heavy.
For a lighter texture, combine rice flour with the cake flour.
Add an extra egg and a little more oil, or butter to add to the moistness.
You might also add some dry milk powder to the cake mixture, as this adds flavor and when mixed with a little water, helps the cake retain moisture.
OVEN TIP:
When baking the cake, place it on the middle rack... On bottom rack, place a pan of hot water.
This helps create steam in the oven, and helps keep the cake moist.
Have fun, and please do let me know how you made out with this, okay?
Elaine