Just took a test ride with the normal Fleshlight case and it is also working. No need for an aditional adapter 
You just have to build the loop before you insert the case so you can make it a bit tigher and it gets stuck if you push it in with force. If you just wrap the velcro around the case then it will be too loose and it drops out. Push the case until the velco loop touches the top threads so the friction is maximised.
The additional weight slows down the Handy but if you use it with slow or medium speed scripts then this works quite well.
I have not tested this in vertical position yet. Maybe in a few days…