DIY rubber/soft fins for grom

I’m buiding a couple hybrid chip/wake/beach boards for my 6 year-old niece and wanted to incoorprate some soft fin setups. Anyone know of a good matrerial/construction for this? Or possibly a good manufacturer who specializes in soft fins?

 

Thanks.

 

http://www.surfcohawaii.com/Pro-Teck-Fins-s/26.htm