AFAIK Polyurethane is pretty malleable
So the first problem, if you try and cast the existing threaded holes, the resin will not fill the hole. Leaving you with a standoff that has just a little indention in it, if you use a self threading screw, the standoff itself will explode trying to displace the resin. Resin will flex a little, but not as much as you need to make it work. You have to drill a hole then thread it. Beleave me i tried it.
Second problem, Resin is pretty malleable, leading to a case that deforms fairly easy unless its heat cured. This could be remedied by testing tons of different resins, finding one that has the rigidity that you need, but it may not be clear, etc. There goes your budget (I tried this too)
Third problem for the HHKB, the caster will need 2, 2 part molds because the HHKB case is 2 parts. This sounds easy but in practice its very hard to get the consistency you need out of resin to make the 2 halves fit together properly.
If he finds someone that has the tools and the know how to pull this off, it will not be $60, it will be double that, easily.
You need injection molding to pull this off with consistency.