Damage Dice Roller Rules Yeah!
All damage dice should be rolled on the table, if you take 1/4 your total hitpoints in an area then you take disadvantage on all rolls that utilise that body part. Crit dice always roll maximum, but the location they land in ceases to work, and you will bleed to death in the number of rounds as indicated on the die unless the wound is staunched (cost an action or something). If of course if you can land a crit die on the head - its game over.
This is a5 size, I considered a4 but thought it might be a bit big? I like the idea of the head being a smaller target and rewarding a steady calculated die drop.
You could make a bunch for different creatures - a beholder would be cray cray.