Here's some pics of my snakes being weighed. I got a set of digital scales from Woolworths, these are Salter, but any shop that sells cookery equipment should have suitable scales.
For the smaller snakes I put the tub on the scales then turn them on and pop the snake in the tub. My ratsnake has to go in a pillowcase because he's too active to get an accurate weight, then I take away the weight of the pillowcase to get his weight.