I've bought 4 of these, 2 pair. I wear men's size 12D, but I got one pair of these in men's SMALL and another in men's MEDIUM. They don't fit as advertised. First they are cigar-shaped or tubular, rather than more flattened like a foot or all shoe trees. I phoned the owner about this, but he insists that round tubular is how most of his customers feet are. I know for a fact that flatter like shoe trees are sold all around the world for the average foot. I own 20 pairs of shoe trees in 5 different brands. (I'm a shoe guy! :-) ) Also, I learned from my shoe repair man who has some of these. He downsizes to a smaller size because they can expand to stretch the toe box, without also stretching the arch and mid-shoe that needs to fit snugly.
Anyway, I bought a second pair of these, despite fit issues, because the materials, wood and metal, were better than the British brand at my shoe repair shop. The British brand actually was much more ergonomic & fit normal feet, but I didn't like the hollow plastic foot shaper. Also, I couldn't sand or grind the plastic stretchers to alter the shape if desired.
This Footfitter brand without modifying would actually destroy Slip-Ons, because there would be no open top with laces to cram these into. To make the Footfitter brand fit, I first used a black pen to mark how much I wanted to take off the top & bottom. I used standard shoe trees as a rough guide, but still left these Footfitters marked for thicker or higher. I used a belt sander with coarse 50 grit to remove the bulk of excess wood, then a finer grit to smooth it, and finally some lemon furniture oil to finish the raw wood.
Then there was the problem with the drilled holes to put the spot-stretching plugs into. There weren't nearly enough pre-drilled holes in the right locations to place the plugs where you might want. Approx. 5.5 mm holes were perfect for the plugs, but I had a 5mm (too small) & 6mm bit (too large). I used a 1/4" drill bit, which was slightly too small, but wriggled it a bit sideways to enlarge the holes. I also slightly drilled/modified some of the pre-drilled holes which were haphazardly drilled at incorrect angles. The plugs when inserted did not sit flat.
In the end, I have 2 pair of stretchers with high-quality materials. I expended a great amount of time modifying incredibly sloppy quality control and design. I like the end result, but question whether it was worth all the time. If I ever decide on a 3rd pair of stretchers, I would probably try the plastic British-made ones in my local shoe repair shop.