OK.... here's a more detailed explanation of what I think will solve this issue for you (hopefully). I don't have all your exact measurements but hopefully the concept is sound enough that you can adapt as needed.
A) This is that small area where you put the tape above your button. It looks like maybe 1/2 inch tall by an inch or more wide. I would route this area by 2 or 3/16ths and glue the wood pillar to that. This way you're getting more strength than just a butt joint since you'll have some side area to glue to, making it more of a rabbet joint.
B) This is the pillar. Conceptually it could be made of anything strong. The idea is you want it stiff and straight though so it's always perfectly aligned. I thought about metal for this which could be screwed in at the top and bottom, or perhaps super glued. I like this better than the wedge at the top and a long bolt because it eliminates the problem of exact bolt length and awkwardly trying to get a really long bolt lined up from the bottom.
c) This is a 1.5x1.5x.75 block of wood (pine). I'm using these everywhere in my build with threaded inserts. Gives you plenty of room to accurately drill a center hole for the insert. The bolt length isn't critical because excess can just stick up beyond the threaded insert a bit. This is glued to the pillar, again ideally with more of a rabbet joint.
D) I couldn't tell how the front was anchored down, if at all, so this is an addition to add a single support for the front and an additional force to help the top from popping off. Again I would inset it with a router 3/16 to make a rabbet joint for extra strength.
E) Under your cab, you inset 2 holes that are big enough for the bolt and a washer. Keeps the bottom of the cab flush.
F) I think this might be an even better option if possible. I moved the concept to the sides and simply cut out rectangle areas from your side braces to make room. Then you have anchors on both sides that should keep it nice and snug.
You might also be able to make a slide in catch on the lower front to hold that part down instead of another threaded insert. Anyway... hope this helps get this project wrapped up for you.