Over the last couple of years what I have come up with in constructing "built-up box" fuselages from stick balsa is to lay down the two longerons over the side view, with pins just off to the side of the uprights (not in line with the uprights) so that I can lift the two longerons and slip the "upright" strip under them.
I then mark the top end of the strip to get the proper angle of cut, remove it and cut through. It is then re-inserted under the bottom longeron with the top longeron pushed down in place and the upright strip is now marked (on the inside of the lower longeron), removed and cut. With a little practice the exact length/angle is achieved very quickly.
I have made up a couple of "tools" to speed things along. One is a popsicle stick (1/16" x 3/8" x 4 1/2" approximate length) with a 1/4-3/8" length of razor blade glued on one side with about 1/32" projecting from the squared-off end of the stick. This is the "marker". Just bring the blade side of this tool up against the inside edge of the top longeron, hold the tool vertically, and press down to mark the angle of the upright. Remove the strip, cut, and insert the strip again, then mark the bottom end for length and angle. Diagonal pieces can also be marked for angle and length with this marker tool.
A second tool used in conjunction with the above is the same popsicle stick material, made up of two sticks, again with the ends squared off and with a U shape, say 3/16" wide by 3/16" deep, cut in one end. This opening is good for 3/32" and 1/8" strip. The two sticks are epoxied together with a length of thin razor blade glued between them at the opening, with 1/64" or less projecting from the end. I use this tool to cut through the strip after it has been marked. Rather than pushing the tool straight down, a better result comes from rocking the tool from side to side to get more of a cutting motion. The long handle makes it easy to keep the tool vertical. Depending on the angle of the cut, it may be necessary to use a regular blade to finish a cut on diagonal pieces.
Finally, instead of just using pins to hold the longerons in place, I have made up a bunch of 1/4" square by 3/8" long blocks, which are pinned down along the outside edges of the longerons. The block for the top longeron is pinned down exactly in line with the upright, so that when a strip is pushed up against the top longeron, a bit of pressure can be applied to get a nice, snug fit.
The blocks for the bottom longerons are pinned just off to the side of each upright position, to allow placing the strip material properly. I use just pins at every second station, on the inside edges of the longerons. I do not build one side over the other, as the pins/blocks remain in place; and with a little care two identical sides are arrived at. There is no messing around with separating two fuselage sides. This method works beautifully and results in almost automatic success.
By the way: I live in Eastern Canada, in New Brunswick. What I utilize for blades is used Trac ll or similar, or even DollarStore blades. These blades are a little over 1/4" wide. It is preferable to use blades which have some sections with no holes. Some blades are punched through with so many holes that what is left is quite flimsy. These thin blades can be easily cut with scissors.
For gluing the blade to the marker tool, I generally use CA; and for the guillotine I use epoxy (which gives more time to make sure the blade is lined up properly). Make sure not to leave any excess epoxy in the U section on the blades! I cut through the strip material on either a cutting mat or else an end-grain block, say 1" square by 1/4" thick, so as not to dull the blade. I have not found it necessary yet to make up a bunch of these tools, but it could be a good idea to have a few spares handy. But I did make a metal holder which I epoxied into the end of the tool, and I simply insert/remove a section of blade using a pair of needle-nose pliers.
The 1/4" square x 3/8" blocks can be of balsa or some harder wood such as pine. I butt one end of the blocks and not the edge against the longerons; so it is best to place the pins 1/8" from one end, rather than in the middle of the blocks. The pins stay in the blocks after they are removed from the plan. Of course the ends of the blocks must be at 90 degrees.
The main point of my technique, of course, is inserting the upright material under the longerons to get the angle of the cuts and the length very near perfect. It is not absolutely necessary to use the 1/4" square blocks, but I believe they do help to get a better result.
I also used this method to cut the cross pieces for a Moffet Winner. I just laid down two temporary longerons on the top view. It's a good idea to mark the cross pieces with a soft pencil to make sure they are glued in place in the same position that they were cut, and not rotated 90 degrees, or even front to back.
Tail surfaces can also benefit from this idea. Just insert the "rib" material under the leading and trailing edges, etc.
This is long-winded, but is much quicker to do than to explain in writing.
