I have a 3/8" u channel on the top of my pony wall which is the only glass I have in my shower. The glass place insisted that there was no such thing as a 3/8" u channel and tried to sell me a 3/4" but I googled it to show them otherwise and pushed them and they ordered it. It looks great and is much less visible.

I'm not an expert in transoms or steam shower design, but I'm surprised nobody has yet said why they were invented, get installed, and are used. 1/ Steam machines make steam. More and more of it. Without new steam, the experience of a steam shower can be less and less enjoyable. New steam is needed. It actually puts pressure on the walls, and in 24hour commercial steam showers this is a big thing. But, even at home, where does all the water vapor go? It doesn't all become liquid (condensation) and go into the drain. 2/ :: Plan where it will go, otherwise it will go places you don't want it to go. A transom is a safety valve putting your excess steam into the bathroom, from where the fan exhausts it. 3/ :: Otherwise steam seeps into everything. Wall-ceiling junctions. Light fixtures. Into the attic insulation. Water vapor can penetrate anything, because it has time on its side, and the world's best solvent: H2O molecules. The opposite approach is "If you only use the steam shower for a short time and not often, who cares?" Anyone want to defend that point of view? I think there is no point building something that you "really ought not to let run". Some future user might want long steam showers, or frequent steam showers. Without a transom, you are guaranteeing that your escape route for water vapor will be confused, since it's unplanned, uncoordinated and unwanted, and your result will eventually lead to problems. As far as I know. Anyone reading this thread so far would be well advised to look more into it instead of concluding that transoms are just for fun. Many homeowners don't care much about codes or practices and will allow anyone to build anything for them. After that, it may or may not work as expected, and ultimately the homeowner may or may not be satisfied. This applied to all fields of construction. My writing this out is not to say that anyone here has been intending to do the wrong thing deliberately. Hth

You can use a u-channel and there are several really wonderful examples of showers that have been installed with them. TreasureTheDay's stands out as a recent example. But you can also use clips and it will be just fine. We installed a steam shower about two months ago with a set up very similar to yours (fan outside) and used clips with silicone sealing the space between the glass and solid surface. Though it's been hot, we've used the steam shower at least once a week -- it's great for aching muscles after a day of gardening! Your glass installer is quite right that no residential steam shower enclosure is perfectly steam-tight. But even with our transom, we have minimal leakage. It is not a problem. It's tough to get far enough back to get a picture + glare from lights, but here's ours. From Bathroom

millworkman- The biggest concern is for horizontal or near horizontal surfaces, like curbs. I have one shower that has a track at the bottom for the sliding door. It rests on the curb in a bed of silicone but is only mechanically attached at each end with a clip that has a single stainless steel screw into the wall, and the screwholes and screws were are well-siliconed. The problem is that if you penetrate a water-proofed, horizontal surface, it has to remain perfectly caulked forever because water does not drain away quickly as it does on vertical surfaces. That is one of the reasons I prefer to see clips used only on wall surfaces, and to have the bottom edge of the glass siliconed to the floor/threshold.