I have one of those Wheeler brand levels that rests on the raceway. It seems to be working fine since I bought it even though it is cheap semi-flexible plastic....I've only done 6-8 scopes with it though.
I use the Wheeler kit as well . There are other techniques to use as well . Check out scope installation by Ryan Cleckner at min 8:36 using the pic rail and a flat piece of metal.
Use this, and don't worry about the rail level if you can't access location. Having the center-line of the barrel plum with the scopes crosshair is the critical measurement.
I have used the EXD tool and use the Wheeler tools/levels as well. If you use them both at the same time you will get totally different indications of what is level. Throw a bubble level on the pic rail or on a scope cap and you will get even more confused.
The EXD tool is the only device that makes sense to me but the other methods should simultaneously indicate a level rifle and they never do.
I've used the wheeler tool but found that the rails aren't parallel font to back. Instead, I pull the bolt out and put the level on the flat part of the tang. I did this method on the last two guns I put scopes on and the tall target tests indicated that no further adjustment was needed.