18"? this ain't april 1, man. anybody have the measurement?
using what looks like 48.2 inch height from a post here, a side on photo, and a scalar i estimate about 30 inches. i will measure the height on the spyder later. might make for some yucks.
at least you have the diagonal measure due to the inset of the top to the outer edge of the sill going for you rather than the vertical measurement i approximated here. that should be good for a few more inches anyway. everybody go out and practice crawling through a 30-36 inch gap