>>10908323You have some options here and it really depends on your person preference, time, and money.
If you make it yourself, you'll just have to accept that it won't look neat or perfect due to your difficulty with making things straight. If that's okay with you, start with a premade poncho and use the method that is going to be easiest for you. Ribbon with sewing glue or WonderUnder or fabric paint would work. It's just preference. Sewing would've been the best, but clearly that isn't an option for you. Also, don't use knit. Use a non-stretch and not-holed fabric like cotton. I'm not sure why you'd need waterproof necessarily.
If you do not want to make it yourself or want it to look better, try seeing if you can find the cosplay online. Some people sell their old costumes and some companies make and sell them. Or you can commission the pieces from an artist if you have the time and funds.
Good luck, anon!