Firstly, I suggest a smoke bomb, not something that damages people. It is a weapon to be used defensively and for misdirection. Not for offensive purposes.
Secondly, I know the firebomb already makes some smoke and I don't believe it burns anything.
Thirdly, reduce the weight of the smokebomb, aka firebomb, from 62 to 3.
Fourthly, increase the quantity from 1 to 2.
Fithly, increase the quantity of smoke slightly and the length of time it smokes. 4 second duration, 2 meter cube roughly.
(To see how minimal the amount of smoke is, try the firebomb)
Sixthly, must not be wearing armor heavier than lamellar to use.
Seventhly, increase cost from 900 to 6256.
Eightly, increase the PT requirement from 1 to 2.
Ninethly, cannot be used on horseback.
Tenthly, cannot be used during rain (don't know if this is possible, if it is, rain could also reduce the effectiveness of bows and xbows too).