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Here's what worked for me, many years ago. Before you even start off on the flight, adjust your seat until you see the top of the cowling without straining your neck...make sure you are positioned correctly for reach as well, so when you are sat comfortably, able to reach everything and being able to see the top of the cowling, you should be good to go.
Once at cruising altitude, trim for straight and level, preferably over flatish terrain, confirm that the aircraft is maintaining that attitude and make a disctinct mark (I used a chinagraph pencil) on the windscreen in front of you, just where the sky meets the ground, off in the distance. It makes a really good attitude reference for the rest of the flight...and it wipes off afterwards!
Later on when converting to rotary flight on a Bell 47, with a bubble canopy, that method came in really handy. Some pilots I know, still have a piece of tape stuck there all the time.