If there's enough distance between the top and shelf for your router, adding the grooves for the dividers is easy as long as the back isn't permanently attached yet (I'm assuming this from your post). Make an accurate T-Square out of a couple of scraps, set the base against the edge of a scrap board and rout through the end - this will give you an exact location for the groove and, as you're using (I think) 1/4" hardboard, a standard 1/4" bit will work. Lay out the location of the dividers on the top and shelf, set the T-Square in place with the edge of the slot lined up with the mark, clamp the end so it doesn't move and rout your slot. With no back in the cabinet, you can flip the T-Square end for end and rout from either side as you get close to the sides. As far as the dividers for the drawers in the lower area, make a block of the appropriate width, slide the divider up against it to fix the location and nail it in place - it's just a shop cabinet, a little glue and some finish nails is all you need. All my original shop wall cabinets are built with the tops,, bottoms and shelves glued into dadoes/rabbets and held with narrow crown staples and they've held up for 20+ years - wish I'd put adjustable shelves in them now but that's another story. Mostly use pocket holes now but still use dadoes and staples for a cabinet that's a little unwieldy.