My original idea was not good because the kings on a3 and h3 could be put in check right on the first move. This one is better: pawns on a3 and h3 and kings on a2 and h2.

But 10 x 10 sounds even better.

As to pawns promoting to kings: I proposed it simply because I saw no point forbidding it when there are several kings on the board anyway. However, if it were allowed, there would be a problem in positions where a promoting pawn can capture the opponent's last king. In that case, the player on the move would win, no matter what number he'd throw. For instance, in the position W: Ka1, Pb7; B: Ka8, Nh8, if black on the move throws a 2, it will be certain that white can capture black's king on his next move, no matter what. To give black at least some chance, as I think he should have, there should be some kind of a limiting rule, for example: the opponent's only king may not be captured by promoting a pawn to a king. In that case, the other player would still have a 1/6 chance to survive, as he has in standard Dice Chess.

Or if you think it makes things too confusing, we could keep the rule that pawns are not promoted to kings. Promotions are very rare anyway.