Haas pedals look the best and I am using them for inspiration, they are +£27 a pedal on a UK site...
The plan is a Zefal toe clip and bungee cord, with or without webbing, to be decided. I can ride for hours on my tadpole but when you try and relax you have to keep your feet on the pedals and I get cramp in my calf muscles. The bungee cord should enable me to take pressure off without my foot falling off the pedal.
Otherwise you could look at good old fashioned road pedals with straps. They only work well with narrow shoes from back in the day though, not the comfortable Decathlon walking shoes I like to wear now.
There is a new wide strap pedal that looks interesting, you have to twist your foot to get it in or out.
If I had a 3D printer near me I would try and prototype a sliding heel support with tension spring (or bungee cord). That and a Zefal toe clip would be integrated into my ergotec pedals
https://www.ergotec.de/en/products/pedale/sub/city-trekking-pedale/produkt/ep-1.html
If my prototype works on ordinary pedals I will go for these
https://www.ergotec.de/en/products/pedale/sub/city-trekking-pedale/produkt/ep-urban.html Ergotec pedals are so comfortable and the most slip free pedal I have ever used. At 47 € a pair a long term investment!
For the moment my bits are lost in DHL logistic hell...
3 days from Germany, 4 days and counting in Biarritz, 21 km from here