There are a bunch of great designs out there that seemed a bit over-built. My design is not yet tested, has a few potential weaknesses but was simple to build and should be simple to maintain. The most likely point of failure is the para-cord so I recommend carrying plenty of spare cord.
I ordered the sled, recommended in several posts, from Midland Hardware for about $35:
The Pro Expedition Sled 60 x 20 x 6 seems durable and rugged without being heavy. It has two nice rope eyelets that worked perfectly for attaching the carabiner. It seems like a good all-round sled.
I bought a 10’ piece of ¾” PVC and cut this in half, sanding the inside of both ends so that no sharp edges attacked the paracord line I ran through them; $3. The line is doubled; probably excessive but added no cost.
I bought 4 carabiners at the local Ace hardware for $1.50e. These are not for climbing; those cost about $7 bucks each. These have “SAFE WORKING LOAD 150 lbs” stamped on the side but don’t seem that tough.
I bought a spool of paracord for $10 (the good stuff rated for 550 lbs) and a “webbed pistol belt” for $20 from the local Army Navy store. I have a lot of spare cord and used maybe 30′.