Toe clips are not exactly "heel support" that rider may need to secure feet on recumbent trike.
If you read the post you'll see she is asking for something she can slip into easily. Which is where the cleat part of a toe clip is the problem, No cleat and half-clips are decades-long solution for casual riders in normal street shoes to solve the issue of feet slipping off the pedals.
Actual heel support is an entirely separate issue. For that, what you are really asking for is arch support. A mid-foot position and a longer pedal does this. A mid-foot position allows pedaling to be a stomping motion that takes away the forward push that causes foot slippage. It also eliminates foot pain on aging arches. You do that with a longer pedal.
The absolute star of that show is the Pedaling Innovations Catalyst. But post-COVID they are $150 for a pair of them. I use them on all my flat-platform-pedal bikes. You can plant your whole foot on them essentially. Originally designed for mountain bikes, they work just as well in less-demanding applications.
Oversized flat pedals designed to rewrite the rulebook
www.bikeradar.com
The Catalyst Pedal’s unique design is based on several scientific studies. J.R. Van Sickle Jr, M.L Hull/ Journal of Biomechanics 2007 – This study showed no difference in power or economy between pushing through the ball of the foot and the mid-foot pedal position. They thought that there would...
pedalinginnovations.com
The difference in foot comfort is amazing. But they are $150. Here is the next-longest pedal on the market at only $39. Not as long but especially for smaller feet, should do the job.
Look closely at both pedals. They both have grippers on them. Grub screws on the Catalysts and tiny blunt spikes on the Rockbros'. Wear athletic shoes with these. The best on the market are the Adidas Free Riders. You can actually stick to the pedal a little when pulling up and off it, their grip is so good. There are similar shoes that will work, and a good soft-soled running shoe will do it. Shoes like the Free Riders will not suffer any damage from repeated use with the pedals.
But the $9.70, 10-minute solution is a set of Zefal half clips. A design that has been unchanged for God knows how long.