How many of these do you need to manufacture? It makes a difference to the best method.
If the volume is small (less than say 100), 3D Printing might be an option. More than that, injection moulding is the best option - but the cost of the mould tooling is likely to be in the $1000 to $10,000 range. Fine if you want to make millions - otherwise expensive.
A third half-way solution is to cast in Polyurethane, which can be bought as a two part mix. (There are many alternatives to PU which work the same way) Mix the two parts, pour into a mould and wait for it to cure. The mould can be made out of Silicone. Recently, I've 3D Printed a positive blank then used this to make a Silicone mould. Each Silicone mould is good for about 50 parts before the surface finish degrades. There are many PU Moulding companies who would do this for you. The tooling cost is lower than Injection Moulding, but the material & labour cost is higher. This works well for medium scale manufacture (100 to 10,000).
My approach for all of these would be to use CNC Buss Bars (Brass?) with the threads cut in those. (For smaller volumes, the Buss Bars could be laser cut and the threads tapped manually.)
One option would be for the buss bar to slide into a groove in the plastic housing. The housing would have two pockets, one below each screw. When the screws are screwed through the buss bar, the threads extend into the pockets, preventing the Buss bar from sliding out of the housing.
Threaded inserts or even plain nuts could work. Generally the approach is to insert them from behind into a pilot bored hole, so in the event of over-tightening, it is compressing the plastic between the nut & the buss bar, rather than trying to tear the insert out of the plastic.
Pressing a nut into a Hexagonal pocket is often used in 3D Printed parts to give a strong thread.
