They are designed with a central pick up, this means they don’t mind which way you mount them, in the Rebel it shortens the lines. Since we use a felt hi-flow clunk there is no air in the trap on landing, so it was only the first fill we needed to tip it on its nose to get the nipples to the highest point. The test flight took off with a 1/4” dia air pocket bubble at the top of the MAP and landed 7 1/2 minutes later with exactly the same!
I would not mount a BVM bag type flat like this, only angled, the MAP trap is not as fussy.