DIY convert ups

[Inneedofcuddles]

I’ve been looking at buying convert ups but they are quite expensive and would take a good while to ship to where I live. Does anyone know how to make diy convert ups? I figured I’d need some Velcro elastic straps but I’m not entirely sure what id do, any help or guides would be appreciated!

 

[Sheepies]

I've tried to sew my own with no luck.
Best solution I've found was to use fitted sheet fasteners. Ideally adjustable ones.