Maybe a long answer but here it is.
My Trasharoo lasted about five years. I always took it off after use and washed the dirt off with soap and water. Sounds a bit anal but worth while. The people that complain about the short life of the Trasharoo usually are leaving on the truck all the time.
I noticed the liner in the Trasharoo would flake off after a wash or two. It seemed to be attached to the nylon with a sort of adhesive.
I've noticed in promotional pictures or pictures of people loading up their Trasharoo with all sorts of items. The rear of the truck is the dirtiest part of the truck getting covered in dust or mud .
I also have a MSA 4 X4 trash bag. It is made of a cotton/Poly fabric. The only way to use this bag is to put a rigid plastic bin inside. The bag cleans up nicely and has lasted longer than the Trasharoo but I don't like it as much.
I have seen the Howling Moon bag which is made of a PVC material . I like that it keeps it's shape I suppose that would minimize the dreaded Trasharoo sag. That would be the easiest to keep clean but I don't know how one would suspend a trash bag inside it.
In answer to your question.
A perfect textile, external, trash carrier would have the following features :
Weather resistant material heavy nylon . Able to be washed without falling apart.
Flap that easily flips back for easy trash loading. No zipper pocket. Loading up the flap defeats it being easily moved out of the way.
Either a lip or other manner to suspend garbage bag inside. I use binder clips to keep the trash bag in the Trasharoo. I thought having a metal frame in the top of the bag that can be pulled up and secured to attach the garbage bag and would also give the trash bag some shape. Something like the last picture.
A three point mounting system to prevent sagging. Even crossing the straps does not prevent sagging. I've thought "D" rings on the bag three across the top and botom and one pair in the middle and using a ratchet strap to tighten the bag would be better than the fastex buckles which always seem to slip and break.
A strap around the middle would help keep it in place.
The MSA bag has an "anti-drop harness" that keeps the bag in posistion.
A couple of fold down ( like a dry bag) side pockets with buckle for extra garbage bags. The fold down feature would help keep out dust and rain.
Of course a couple of rows of MOLLE for some customization .
I ordered the G.A.R.B. bag and will see if it worth the money.
I hope this was not too confusing
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