I have 2.
One made with a key ring, make sure you put some tape over the end of the link as this will rip any 'ranger band' (bike innertube cut up to make elastic bands) you use. You can slide this down the arms when wanting to use conventionally and it won't interfere.
One made with a brew kit bottle cleaner (came about 3 feet long so had much more to play with to get right) this is kept in place with a large 'ranger band' so is moveable if need to use as a catty.
Which is best? My 80lb self loading crossbow;-) seriously though, performance wise, negligible difference between the ring and home made whisker biscuit, both can be used as a normal catty, ring is quicker up & down.
The flights need to be reasonably small for the ring otherwise they tear easily.
Oh, and before you start replace the basic tubing with black therra bands. 5x times stronger and any stone or arrow will be much more deadly.(not that you can hunt with the arrow here in the uk though)
Hope that's some help.