Thomas Brodie-Sangster has always had a really Eustace-y look to me,
coracle. I think he's probably much too old as well, although I've considered that at least you wouldn't have to worry about him aging too much in between SC and LB!
There was a recent film released called
A Monster Calls (ironically with Liam Neeson voicing the monster) that starred a boy named
Lewis MacDougall, and he actually looks a
lot like how I picture Eustace — at least, when my imagination is being influenced by the Pauline Baynes illustrations and Will Poulter's portrayal of Eustace isn't sneaking onto my mental stage. Unfortunately, MacDougall will be fifteen in June and he's already grown up a bit as it is, looking at
pictures from the recent film premiere, so I doubt that he would work. (Unless the filmmakers are intending to make Jill and Eustace fifteen or sixteen years old, and I hope they are not!)
All that said, I wouldn't mind if the filmmakers went with a Eustace who looked quite a bit different from Will Poulter. I used to want more visual continuity with Poulter's performance, but as the years have stretched on, it seems less important — especially if SC's Eustace looks younger than the Eustace of VDT. (Will Poulter was sixteen during filming and looked at least fourteen, imo.)