After some thinking and research I decided that 3D printed metal would be the best way to widen the track width of the front wheels on my Bruiser. So I opened openscad and designed what I call a hex extender. It is designed to match a 12mm hex and has a 5ø bore that allows a 5mm M4 barrel nut to fit in.
Before this can work, a hex conversion such as the RC4WD kit is needed. Then I used barrel nuts from a kit named: Aluminum Alloy 12mm Wheel Hub Hex Drive Adaptor +10mm Offset SCX10 WRAITH Black i bought off ebay. It might also be possible to use longer wheel axles. I didn't experiment with that
The barrel nut fit very tightly into the extender so I ended up using a needle file to enlarge one of the extender bores. Further I put some grease in the bore of both extenders to avoid binding.
The 3D print was done by i.materialise.com. They claim that the strength is the same as regular aluminium. I didn't test it though. It is available from their WEB site here: https://i.materialise.com/shop/item/hexextender-v2
Here are some pictures: