If you are making a 3 string or 4 string model, you really don't need to reinforce it, assuming you are using a decent hardwood like oak or maple.
If you really need to reinforce, you can put a commercial truss rod in your neck. You can put a piece of threaded rod in a channel. You can put a piece of carbon fiber in a channel. You can laminate the neck from multiple pieces of wood, with a fingerboard added afterwards.
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Try a piece of steel flat bar epoxy glued into a groove routed in the neck
try adding a 1/4 inch fingerboard....that should help.
If you are making a 3 string or 4 string model, you really don't need to reinforce it, assuming you are using a decent hardwood like oak or maple.
If you really need to reinforce, you can put a commercial truss rod in your neck. You can put a piece of threaded rod in a channel. You can put a piece of carbon fiber in a channel. You can laminate the neck from multiple pieces of wood, with a fingerboard added afterwards.
Lots of ways to do it...