Hi Gus, We haven't done any E/G mixes yet but we have been working on a frame system that uses reinforced concrete mix. Check out the thread titled "Ultimo". As a matter of fact I am just about ready to get back to work on it.
I have looked into Epoxy granite and while good for big builds I dont think its right for Vslot voids. What I would do is cast a EG base to then strap the Vslot to its sides for linear rails. This would make the base heavy and dead and because of the size of the casting you can get a much better spread of aggregate size too. To to EG well you need to run a model to work out the best spread of different aggregate sizes/ratios, then you need to find a good resin system with long enough pot life, wash and dry your rocks, mix the resin into it (get your packing model right and you can get down to very low resin %) and then vibrate the whole thing to pack it and remove air. Vac bagging and infusions are not really reliable. The resulting block is amazingly dead and you can cast in many features and inserts for threads Once again the above method is best practice, higher resin ratios and non vibrated will work well but may result in a few voids but if you keep the shape simple its unlikely to be an issue any way Concrete is a far simpler and achievable btw.
yes agreed concrete will shrink when cast in but my point was NOT to fill the voids in the Vslot stick. The amount of added mass you may be able to pack in is relatively small compared to casting say a 1000x500x50mm base used to support and dampen a vslot frame. If you are really keen to fill the Vslot I would block of each end with say a printed plug or the end caps with a layer of sealing foam and then filling from the other end before capping. Good luck trying to do that with an epoxy slurry AND keep it all off the surface your wheels are going to run on.....