All you need to ascend a rope is a pair of locking carabiners and a prusik loop. They stow pretty easily on your harness:
I had the best luck with asymmetrical "D" carabiners. I also used 6mm prusik cord which worked nicely on my 9mm rope. Getting the length just right took a bit of experimenting. It ended up reaching just above my shoulder when I stepped on it:
With a three-wrap prusik hitch and a girth hitch around my foot, it is just long enough to give me room to grab the rope above the rappel device and below the prusik hitch.
Instead of a second chest prusik, you can use two carabiners with a garda hitch. It will act like a ratchet. This is what the garda hitch looks like loose:
When you pull the slack through with your brake hand, the hitch walks up until the load strand wraps around the spines of the two carabiners causing them to clamp down on the brake strand. Try it sometime. It is rock solid!
It's a bit hard to photograph yourself ascending with a garda hitch so you'll have to trust me that it works. The key is to take big bites with the foot prusik. Wrap the brake strand around your hand so it doesn't slip and haul the slack through powerfully as you stand up in the foot loop. It takes practice. After a few trips, I rappelled halfway down the huge 20-foot cliff I found, tied myself off, set up my minimalist ascension rig, and ascended back up. You need to tie the garda hitch below your descender, obviously:
This can all be done with gloves on. From here, you have to stand in the foot loop while you remove the rope from the descender. This is a busy task so no photos. Once your snug against the garda hitch, you can take a break and shoot a photo of yourself hanging five feet from certain death.
I wouldn't want to haul myself up El Capitan using this rig just because of the friction pulling the slack through the garda hitch, but as a self-rescue device, you really can't beat it. It's cheap and it doesn't tear up your rope.