Table cloth/ shower curtain will. not. work. It is just a waste of time and energy- they fall apart, bunch up, etc (been there, done that)... Not only that, if you sew them into the dipe, your kid's skin cannot breathe, setting them up for yeast infections, diaper rash, impetigo...
Go to Wal-Mart, spend $6 on a pack of "plastic pants"-- they should last long enough to get kiddo through a size. Even these will fall apart, with hand washing and hang drying (ahem, been there, done that). But they only cost $1 each, and are readily available in almost every town-- no shipping charges.
And kiddo can wear normal pants/shorts over them. This would be the equivilant of using a thrifted raincoat for material.
Waterproofing spray is a bunch of toxic chemicals... directly in contact with or near your baby's reproductive organs... 24 hours a day...
Wool covers can be had for next to nothing as well. Pick up some soft wool sweaters at the thrift store (at least 80% wool), take them home and shrink them. Tons of free patterns on the web. Make longies, shorties, or skirties and you don't have to worry about seperate pants/bottoms. Make wrap style and use normal pants over them.