Personally Id just get a holiday. Its not like you need the padding, etc in your hunting clothes.
Failing that, it needs to be a full strength loop so you could take a 24” sling (a loop) and stitch it to the harness so the entire loop is on the back-side of the harrness with justbthe ends exposed on either side. Then the sling would be your “harness”, and it would just be held in place on your climbing harness with your stitching.
@fatlander on MOST climbing harnesses the rear haul loop is NOT full strength, even if it appears to be bartacked. It is meant to hold minimal weight only so if you drop a rope or some gear onto it it’ll hold, but its not meant to hold a person so its not a safety element on the harness.
Do not use this if you arent certain of the strength of that part. Only a small % of climbing harnesses design this part to hold body weight. Its been a very long time since I looked at that particular harness and I dont remember if this is one. Not clear if the haul loop is holding weight the way you have it rigged or if its just holding the cord in place, but wanted to clarify this because if someone is using this loop to hold body-weight its a likely failure point if (mis-)using a harness for a purpose it wasnt really designed for.