I have heard of people trying to do this but have never made one myself. If you are quite handy and have some free time go for it! Just be sure to share some pictures of the finished product;)
They should last about 4-6 years before replacement is required. Not all sex stool companies have replacement elastic for sale though, be sure to check before you buy.
A TPU sex stool like the Fetish Sex Stool has a weight limit of 300 pounds. Sex stools with polyester elastic bands will still support up to 300 pounds but you may sink too far.
You can use a few pillows to raise yourself higher if needed.
Yes! If you have bad joints it is going to be much safer than not using a sex stool. If you go hard there is the still the risk of coming down and bending your partner’s shaft.