Honestly, anything would help those keys as they are too light even for me to get use to.
I did years ago, place 1/4" to 1/2" foam under the keyboard of my Casio WK3800 with double sided tape. It stopped the keys hitting the body every time you played a key. And it gave "some" feel albeit a spongy feel but to me better. I used it as a student and loaner keyboard.
If you can't find the Sunshine - BE the Sunshine!