The PCB of the Ucurrent is 79mm x 50mm.
The holes are 5mm away from the sides. I think this is slightly different than in your proposed case, but you should check that again.
I think its like 0.3 or 0.5mm difference to the PCB, so screwing into the plastic body should still be possible.
And JLC is no different from other manufacturers, if they see that they are not able to manufacture something, they will tell you.
A friend of mine had a bigger board in which he made many slots and JLC discussed the matter with him, because it was problematic for them to produce.