I have the same handguard. And the same problem.
Believe it or not, I bodged some thin foam in there, then took up the space with a piece of shelving I snatched from the IKEA spare parts bin on my last visit there. It's about 90% of the way to being right, but if anyone happens to come up with a 3D printed solution, I'd happily throw some dinero their way...
I designed/3d printed a handguard that utilizes both screw holes for the original handguard. Fits nice and snug. I also designed/printed the stock
Both parts are wrapped in fiberglass for some added strength
