One of the first things you do after skinning the top of the wing is to trim all the excess cloth, trailing edge and start prepping the inboard end of the wing for the "rib". Following the manual I trimmed the inboard end of the wing trailing edge making a nice straight edge from the tip of the spar to the trailing edge. The picture with rough strands of cured epoxy is the before pic. After you make the edges straight you hollow out the foam 2" deep. This area will get a couple of layers of BID cloth to seal up the wing end. I used a hack saw blade set 2" deep in a handle to start cutting out the foam. It was tedious and not very productive. I didn"t want to sand it all out as that would have made a Lot of foam dust. I ended up making a rough in tool from some scrap flooring and a wood block glued together. I drilled a 3/8" hole through it slightly off center. I then wrapped a bunch of masking tape around the drill bit at the chuck and stuck that through the hole in my jig. I trimmed the tape back til the bit stuck out the other side right at 2". The n holding the flooring side of my jig up and down so it contacted the top and bottom skin, I spun the drill bit as I moved the jig up and down and back and forth. that routed out the foam to a uniform depth in just a few minutes. The pics show what I am trying to describe. This area will get some glass bubble and epoxy filler applied and smoothed out prior to the fiberglass lay ups. This area is hidden and doesn't really matter about looks, but it is a good way to practice and get more comfortable with the process.
New kits come with a premade end rib that is installed before glassing. I could have ordered one, but decided to just do it the old fashion way for the learning experiance