another thing i have been playing with is flour to get some weight out of the bag, and it's not as harsh as sand. with cloth make sure to get a strong broom stick to pack it like crazy, it will get hard enough.
you can also try rice instead of sand. i hate sand it too damb hard.
to get a Thai bag to around 150lbs it will take a ratio of 75/75lbs sand and cloth. if your going the flour route it the ratio will be higher for the flour as flour isn't as dense. you can pick up cloth from any fabric/textile distributor starting from $0.60/lb (thats Canadian) or you can hit up any textile place they will be willing to gather up there unused material and sell it to you for a bit cheaper.
yes it will be a little soft, but you fill it as it settles. if you have a bag that is too hard you WILL start to hold back on your kicks, what is the use then????
kick a softer bag packing it and make it firm. use a rolling pin on your shins to make them tough. you are looking for the weight resistance in the back.
"every second your not training, he is..... and he is doing it, just to kick your ass"
This is what I did to fill mine. I used a 4 inches wide pvc pipe and filled it with sand. I had someone hold it in place right down the middle and filled around it with pieces of cloth. You can buy cups to close the pvc pipe both ways so the sand doesn't go all over the place. They are pretty cheap.
Make sure you stuff the cloth around well. It's a very easy process if you have someone to help you hold the pipe in the middle. The result I got was great. Not too soft on the outside and quite heavy overall.
If you need any help let me know and I'll try to give you more information.