I always use a blender to prevent lumps. I have an older Kenwood model which has a removable centre section in the lid which can also be used as a measure. I add the water first, along with any extras - I add a little bran for extra fibre content and some dried fruit - and get the blender running. Once it’s all smooth I remove the centre section from the lid and add the Huel one scoop at the time. No splashbacks with this method.
I’ve always used the scoop and shaker for all the months I’ve been doing this, I’ve never experienced any backsplash. I generally use 450 to 500 ml of water and my scoops are usually a bit top heavy. I don’t slam the Huel in there, I just gently (sort of) tip it out at first until there’s a layer of of powder, and then I just tip in without thinking. Just don’t bang it in, take it easy, and there shouldn’t be any splashing.
Make sure you don’t compact the powder too much to get a level scoop - overfill scoop and shake rather than rub against side of bag. Or use scales and just do loose scoops till you get to required amount…
I have never experienced any backsplash, ever. How are you managing to get backsplash? Are you dropping it in from a height? or are you you throwing it in with pressure / speed?
If you tip the powder in gently from the top of the shaker, it gently falls softly on to the water. That’s what mine does and not once had it ever backsplash.