I have to admit I have already whacked my head on the corners of the two middle cross supports a couple of times. I haven't learned to keep my head down ALL THE WAY across.
The pipe insulation is a good solution. We have some around here that is pre-split. I'm thinking about 1/8" hard board bent along the curve of the arches for a ceiling when the table is up but there are still two outside edges waiting to scalp me.