Well the breaking strength of baler twine is quite high, but if you suck at tying knots it doesn't help!! (My stuff always comes undone, proven by our flying haylage tarpaulin....)
A good knot sealed with heat should do the trick !!!
Sorry if this sounds harsh but I wouldnt advise anything but roundings or two reins. If you want to ride in something else thats up to you, but to put someone else on your horse with anything but correct safe tack is really risky. If there is an accident you really could be in deep trouble. Find a tack shop that has roundings.