Perhaps one could loop tubing until there was suitable resistance for a close pull similar to the iron shoe.
Here is an easy-to-make variation on that idea.
I took some 3.5 inch long open-loop springs, which you can easily find at a hardware store.
In this case, they came from an old lawn chair.
The actual coil is 2 inches long and 3/4 of an inch thick.
On the iron shoe, the coil is 3 5/8 inches long by 3/4 of an inch thick.
I fitted them onto an old set of chest expander handles.
The ones I used are wavy with a back bar, but looped would work just as well.
The starting pull distance is 9.5 inches.
This is, by happy coincidence, the end stretch on the Iron Shoe.
The feel of both devices is similar.
Though the Iron Shoe starts at 3 3/8 inches, maximum pull,
when the chain makes it isometric, is the point of maximum effort.
I settled on 3 springs for starters, which I stretch about the optimal half length.
When you determine how many springs you want to use,
you should squeeze them closed with pliers for safety.
There is room for 2 more springs, or for an external, or for added safety an internal tether, if you want to follow Terry's design.
This also gives you the Iron Shoe's isometric finish.
In fact, I once saw on EBay that Terry's used to sell a short-spring device,
though the one I saw had only one spring.