We just use kitchen scales. The best thing is they're not digital so they're pretty old fashioned and it's a two person job (one to put pig in tray, the other to take pic so can work out weight). It works for us at the minute but I do wish I had digital ones.
I use digital scales with an upside down pigloo. I just pop the pig in and wait for a bit as the weight fluctuates when they move. I am sure that an old ice cream tub or something like that would work just as well.
i have some baby scales that i use now, but have used kitchen scales in the past. i just put them on a fleece blanket but if you have a wriggly piggy, then inside a cosy bed is fine aswell. remember to subtract the weight of the fleece and/or bed though
another tip is to weigh the same object each time before using the scales (something that will never change weight) and that way you know if they're working properly. otherwise you get some weird readings if the battery is going! (as i found out after panicking once! )
my kitchen scales couldn't be set to zero after putting the cosy bed on for some reason so i had to subtract after. the baby scales i can zero after putting the fleece on