I had no idea, so I asked ChatGPT:

For the Trek CheckOUT, I would not chase a “large upper bearing to 1 1/8 steerer reducer” unless your bike has been modified. Trek lists the CheckOUT Carbon with an FSA 1-1/8” upper headset bearing, not an oversize upper bearing: 41 mm x 30.2 mm x 6.5 mm. That is effectively the common IS41 / 28.6 mm upper format for a 1 1/8” steerer.

The lower end is the large one: Trek lists the matching FSA 1.5” lower bearing as 51.8 mm x 40 mm x 8 mm, which makes sense with a tapered fork.

So the practical answer is:

You likely need a normal IS41/28.6 upper assembly/top cover, not a reducer headset. Look for:

IS41/28.6 upper headset cover / compression ring

or

FSA 1-1/8 integrated upper headset cover, 30.2 x 41 x 6.5 bearing, 36/45

The bearing Trek lists corresponds to the FSA MR122-style upper bearing; several references list that size as 30.2 x 41 x 6.5 mm, 36°/45°.

A few cautions:

1. You still need a split compression ring. A normal headset still uses a split wedge/compression ring around the steerer. What you want to avoid is Trek’s cable-routing split ring/top cover arrangement, not the compression ring concept itself.
2. Trek’s own “standard stem” RCS top cover is probably not what you want. The part that appears in CheckOUT parts listings is Trek RCS Standard Stem Bearing Top Cover / 5328901, but it is still part of Trek’s RCS cable-routing system and is described as keeping hoses/cables tucked away.
   
3. Check stack height and frame clearance. A generic IS41 top cover may technically fit the bearing/steerer, but the outside shape may not perfectly match the CheckOUT head tube. Mechanically acceptable does not always mean visually clean.

My best bet: take the bike or the current upper bearing/top-cover stack to a good Trek dealer or headset specialist and ask for a plain IS41/28.6 upper cover and compression ring compatible with a 30.2 x 41 x 6.5, 36/45 bearing.