Use 2m of Dyneema cord (5mm), Spectra cord (5mm), or a Dyneema climbing sling (120cm). These are very soft/flexible/compact materials. They also have zero stretch which will transfer all of your energy to upward movement.
Next you want to tie a foot loop that can fit one or, both of your feet. I prefer a loop that can hold a single foot. Tie this with an overhand on a bight knot, if you have a stiff sole on your caving foot wear. Use a double loop figure of eight or a double loop bowline if you do not have a good sole.
I use a small rated carabiner to attach the foot loop to my hand ascender, securing the cord with an appropriate knot/tie (locked off Italian hitch, figure eight on a bight, clove hitch, etc). Some knots allow for adjusting down the road.
If using the Dyneema climbing sling, I will cut it with a hot butter knife then tie it the same way as a cord.
I have found rigging it this way the set up is light, and flexible. I can also use it as a belt in horizontal caves, for rescue etc.
Avoid having any knots between the foot loop and the attachment carabiner!