I have been checking the MK center lists from last weekend onwards because my local PS (which my son does not attend) will host the event - but only for its own students. Same is the case for 4 other schools locally that hose "private" testing centers. They suggested that I register with other centers that are open to the public - which in itself brings another dilemma - because the people volunteering to run these centers which are open to the public do so on their own time, so might want first preference for their friends and family - which is the reason why they are already FULL - and some of them are math tutoring centers which might try to attract new clientele by reserving some seats for their new students. I did not know that entering a simple competition for a competition loving child was so complicated!
I just noticed a center in Oregon with 470 openings available right now - hmmm... another possibility to consider.