For the Golden Oldie biplane kit I have substituted lite ply for the 1/8″ balsa fuselage parts. The simple reason is that when cut from balsa sheet the cockpit floor and fuel tank floor are long enough to require splicing. When cut from lite ply they can be made in one piece and thus will be more accurate. I went ahead and added the turtle deck formers to the lite ply sheet while I was at it.
As mentioned elsewhere, the laser cutter does a nice job with 1/8″ plywood, so all 1/4″ ply parts are laminated from two layers of 1/8″. You’ll notice on the Golden Oldie plan that there are lots of little 1/8″ and 1/4″ plywood blocks for attaching the struts, the rigging, the aileron servo horns, and the aileron-to-aileron horns. When you buy this short kit, you will receive a considerable pile of 1/8″ plywood rectangles of various sizes, and it’s your job to lay them out on the plan and figure out where they go. While you’re at it, you’ll have to figure out the stabilizer center section as well, which is in four pieces of 3/16″ balsa, to be edge-joined and then laminated to form one large piece of 3/8″ balsa as shown on the plan.
The plan shows cabane struts and interplane struts made of 1/4″ x 3/4″ spruce. My short kit includes cabanes made of 2 layers of 1/8″ plywood, in case you prefer to build the plane this way and save yourself a bit of trouble. It’s your choice. While I was at it, I also included a floor to be set into the top of the struts, to allow the wing to be bolted through from the top, into the plywood floor, with nylon bolts and blind nuts. The joint between the strut and the floor should be reinforced with a bit of triangle stock, as in the photos on this website of the Sporty Forty biplane. You can build it my way, or you can build it with little brackets as shown on the plan.
On the topic of struts, it wouldn’t save the builder any trouble to have pre-cut interplane struts, because the strut length will vary immensely from one finished model to the next due to variations in dihedral angle. So go ahead and cut these from spruce as noted on the plan, or from balsa.