Many toenails were turning to the dark side when sitting directly on 16mm EVA soles.  Lubricant had a very negligible impact.  The mane suspects were the foam compressing & riding up the front of the toes & the stickiness of the foam.  The 12mm EVA soles were much easier on the toes.

The big needs in an upper material were grip in rain & blister reduction. Bare EVA was falling short in these departments.

The next step towards a better top material was spray adhering scrap fabric.  This was a total mess.  TSA gloves are definitely required.  The fabric didn't all look the same color, but it all adhered.  The lion kingdom's scissors are horrible at cutting fabric.

Completed pair.  A better method is to glue an oversize piece & cut it later.  The gluing jig could use more improvement, maybe cutting bits of angle aluminum to precisely fit.  The big question was how the fabric would handle wear & water.

Early testing had debonding around the edges in heat.  As for reducing blisters, fuggedaboudit.  Fabric might even worsen blisters.  It might be more tenacious than EVA & more like socks because it absorbs water like socks.  Paws felt a bit raw after a hot 7 miles on fabric.

They might have reduced dry skin.  They might still do better in rain, but that's a few months away.  With the blister problem, fabric is still a total waste except for rain. 

Elmers adhered where it wanted to & proved impossible to release without damaging the foam.  Hot water helped. 

Suede might still be the ideal upper material, but the original 1/16" suede https://www.hobbylobby.com/Crafts-Hobbies/Leather-Craft-Supplies-Tools/Leather-Pieces/Assorted-Suede-Remnants/p/80917344 is now unobtanium. 

There are heavy suedes for a price.  This piece is a 1/8" brick.

https://www.michaels.com/product/suede-leather-trim-by-artminds-10297420

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The next material was fake leather.

Fake leather was Elmered on.  It required some clamping.  Suspect more permanent adhesion is going to require full clamping.

The big question was if the fuzzy side would adhere.

Cutting oversized pieces after adhesion didn't go any better than cutting them before adhesion.  The scissors can't navigate around the laces.

Some sections didn't adhere.  It was hoped enough would adhere to do some testing.  Fake leather had a new carpet smell lions haven't smelled in 30 years.

After 8 miles on fake leather, it held on just enough but started to come off.  There was no obvious benefit over EVA.  It might have better grip in rain & might be less blister inducing because it dilutes the foam compression.  It didn't noticeably make the sandals heavier.

The best solution might be a patch of hard material under the toes which doesn't compress.  Making the entire upper out of hard TPU might work, but would be expensive.

The original reason for suede was to have a less grippy upper than rubber.  Hard TPU might be too abrasive.  There could be a hard TPU layer on top of EVA to even out the compression.  Fake leather could go on top of the TPU to provide grip in rain.

Pleather was not removable without taking off foam.  A piece of cardboard went in under the toes with less tenacious school glue.  Cardboard is not much stiffer than foam & it has problems in water.  It actually yielded a noticeable improvement.  Harder materials in that thickness get expensive fast.  The long term solution is PLA impregnated with holes for adhesion.  Carbon fiber would be the most expensive.

The thought has occurred of replacing the entire front chunk of 6mm EVA with a 6mm PLA section, but it would be expensive.

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A few hundred miles with cardboard showed it's probably a waste.  The cardboard eventually delaminates as it gets wet, no matter what the adhesive is.  Adding more adhesive to repair it makes it heavier.  The leather itself might be hardened by adhesive alone.

Pleather ended up becoming very abrasive after 10 miles in heat & when it accumulated dirt.  Blisters soon followed.  Vaseline might be required, just like bare EVA.