Sorry but I do not like the Whisper in tank filters for that reason. They do not adjust and the flow intake is so strong that Male Bettas do have a problem with them almost all the time. The
HOB filters are okay as they can generally pull away from them okay but the width of the in tank filters makes it hard as they are nearly as wide as the fish is long.
About the only thing you can do to protect the fish from the current is to cover the intake with some filter padding or something like thin sponge from a filter of some type. Make sure whatever you use has been cleared for aquarium use. Sponge and other things meant for household use contain chemicals to prevent spoilage and will poison your fish.
The other thing is the outake on those filters do not take well to putting things in front of them as they have the "waterfall effect" and do not have much to attach anything to. It makes it difficult to try to slow the flow from the top.
My actual suggestion would be to get another filter like a HOB filter like a Penguin even that you can at least put a baffle on to slow the flow. It has a very small intake tube and even my baby betta fresh from Thailand had no problem with dealing with it when I put the baffle of a small piece of filter pad on the front of it. He never stuck to the intake either. EHEIM makes an excellent HOB filter called the Liberty that has wonderful reusable easy to clean filter media. I love them too and they are FULLY ADJUSTABLE to a small trickle of flow and a river of flow so you can slowly adjust it to
acclimate your betta to the filter and no baffle is needed. The HOB filters also have the added luxury of making your fish more room to swim as the major portion of the filter hangs on the back of the tank outside of the water. The only part of the filter that goes into the water is a slim tube.
Rose