It may take a bit for conversion, so I don't know how viable it is...but I've started using a system where Size determines hit dice:
F 1hp
D 1d2
T 1d4
S 1d6
M 1d8
L 1d10
H 1d12
G 1d16 (or 2d8..there are ways to "roll" a 1d16)
C 1d20
And a bonus is received based on class/type:
Original New HP
HD Bonus
1d4 +0
1d6 +1
1d8 +2
1d10 +3
1d12 +4
This seems to work great so far...though it does take a bit of conversion.