I finally got around to testing both Sleeper 4.0.1 and FlixEnhancer 1.5.3 on iOS 11.3.1 using the latest Electra jailbreak. I can confirm both tweaks are working. My test device was the iPhone 6s which I had previously used on Electra 11.1.2. It also looks like other users who use TweakCompatible have also reported both tweaks working on 11.3.1. FlixEnhancer is also now confirmed working on the latest version of Netflix, which is 10.18.0 at the time of this posting.
Both Sleeper and FlixEnhancer should be available for purchase on Sileo whenever it is released.
Unfortunately until Sileo is released, purchases will not be enabled on the Cydia Store for iOS 11. Luckily, someone has created a tweak that allows users to purchase tweaks via the Cydia Store. Installing the tweak “MakeCydiaBuyAgain” will allow users on iOS 11 to buy tweaks. You can grab that .deb file here. (make sure to download the latest version). If you’re still having issues or can’t get it working, contact me directly and I’ll do my best to help get either of my tweaks on your device.