Lachlan - I'll admit that I haven't seen a glassless carrier in person for some time. I first handled one about 45 yrs ago, and really don't want to dig through all those boxes of enlarger odds n' ends up on my loft, that I no longer need, or leftover from some "been there, done that" experimental phase. I've machined all kinds of carriers of my own, if that was needed. A modernized version of the Carlwen fluid carrier system with registration pins might be a great idea, except that I'm too lazy to repeatedly clean the immersion oil off all the glass and film surfaces involved - that would be darkroom Purgatory.
For 35mm film, a "stretching" carrier could simply use 4 sprocket hole pins capable of each being tightened 45 deg outwardly. A fairly easy machine shop CAD project in this day and age.
I have a friend quite skilled at using his glassless carriers. But he only prints 35mm up to 645, and relatively small prints using a low-heat enlarger. When I operated a 2000W color mural enlarger, it ran so hot that the cooling fan required four pure silicone duct lines; and the fan alone drew more wattage than my entire house. Even all the light seals in that rig had to be made of pure silicone, aircraft grade. All that got replaced by a cooler system, thank goodness.