Thanks for the responses.
There was a problem with my calculations.
I was going by a wrong spec. The Seymour SN-1 Snath on Amazon has a 6.6lbs spec. That must be wrong. I see 2.2lbs and 2lbs 11oz elsewhere. The price though, on Amazon, it's better than on other sites.
So really, the difference between alum/wood snaths seems negligible.
Referring to the first response: Lead shot, steel shot, would be a nice way of making it heavier, but again, based on both types being close in weight, I think adding sand and/or metal shot inside would only make the scythe set up manageable by powerlifters... and who knows if the momentum can lead to deformation of the snath.
Now, if ounces are crucial, the slight difference in weight can be adjusted using saw dust instead of sand.
So far I'm gravitating towards the wood.