Cut the Sides
Okay, now we build the sides. The drawings detail these measurements perfectly. The angle of the cabinet for the top two speakers is 11 degrees. I suspect, the angle is where the “a” comes from in the DIY Marshall 1960a model number. I traced the first one, and cut the second one to the exact same measurements. This way, in case my angle was off, at least both panels would be off by the same dimensions.
Side piece cut off of the square side of the plywood sheet. Cut this, trace a second one, and cut it, so your setback angles match exactly. 
Mark and drill pilot holes for all of your joints.
I cut the bottom and the top panels directly from the dimensions on the drawing. It’s important to make a decision before you cut these how you wish to join the corners. If you plan on a dovetail joint, adjust the measurements accordingly, taking half the width of your plywood out of the length measurement. I simply used a butt joint and laid the top flush, in between the two side panels. This seemed to be the most logical joint based on the measurements. On the bottom, use the same method. I cut the bottom panel to sit within the the two side panels.
Assemble the Frame
I assembled and squared all the pieces to the outer frame, glued, and screwed each section to the other. The next part was to outline where the inner stops would be. These are 1×1 strips that attach on the inside of the frame to 1) stop the back panel from coming in too far, and 2) stop the speaker panels from collapsing into the body. I used 1×1 pine strips for this purpose and they worked great. My only problem was that I didn’t think through the final assembly while building.
Once I had all four of the panels cut (top, bottom, 2 sides) I measured and drilled pilot holes in all the joints. Do not skip this step. When using plywood, and screwing into the sides of a panel, it is very easy to split the panels. Do yourself a favor and drill pilot holes first. These #8 1-3/4″ wood screws worked great with a good wood glue. I got very tight bonds, too tight in some cases (we’ll discuss later).
Add the Inner Lip
Mark a line 3/4″ all the way around the inside of the frame. and mount the strips inside of that line. I did this on the front and the back of the frame, assuming that the speaker panel would be assembled from the front. But, as you’ll see later, it’s impossible to tolex the frame and add the speakers from the front. I found this out the hard way, and had already glued and screwed my strips on the front frame. I had to unscrew, then pry them off, fill with wood filler, and restart. The front does need these strips, but they are flush with the front all the way around to allow for the recessed speaker frame.
Rough frame of the cabinet. 
Setback for inner 1×1 strips 3/4″ all the way around. (5/8″ + 1/8″ for width of tolex.) 
This is incorrect! These front strips should actually be flush with the front of the cabinet, and the rear strips should only be added AFTER the speakers are installed from behind. (learn from my mistake)
So, after a key mistake and a recovery I had a frame and the inner strips built, along with a flush-fitting back panel. I then cut the holes for the handles. This was a straight-forward process that was easy and mistake free! I used the measurements from the drawing which aligned the holes perfectly. It also helps to lay your handles on the marks before you cut, just in case you accidentally purchased handles that are a different size than the drawing specs. I lucked out, as mine were exact replicas of the 1960a handles.
