I recommend Pedicure Helper.

Using a pedicure helper supports your toes, allowing you to apply manicures comfortably.

It seems to be called by various names, such as pedicure helper, toe-holding sponge, etc.

I bought this at Daiso a while ago, but I'm not sure if this design is still available.

Even though the design is different, the function is probably the same, so I think it will be fine haha.

You should try applying it using a pedicure helper too; it is really convenient.