IF you are going that deep in, well, you’re going that deep in, so, to be officially recognized at having a sawdust making addiction, you’re going to need to have some wood and flat stock on hand even after the project is finished, so….
In other words, half the fun of being all in is, being able to walk over, grab some scraps or other stalk and start a project. As such, if you order too much, and have a place to keep the excess, it’s all good.
You need to plan for waste. They say 10%, but, if you’re starting out and not accustomed to measuring four times…..
Look at a cabinet you like. You can do some rough estimates of how much plywood and 1x you’ll need from that.
The average cabinet should be able to take shape using two sheets, unless you’re going to add in drawers and such. Then you may want a couple thicknesses of ply, but a bit of overkill (e.g., drawers with 3/4" bottoms) never hurt anyone. Especially when stacking them with lead and copper.
Do you want this puppy to be able to sit out in the open, with the back visible? If so, you may need to stay with upper shelf ply all the way around. On the other hand, if it will be against a wall. 1/8" ply might meet your needs and wants for the back, and the racks/supports would act as stiffners.
Test drive your imagination a bit. Figure out what all you want this thing to do. Is it going to have glass? Is it going to be halfarsed safe? Is it going to have secret cubby holes that rare earth magnets hold closed? Will it have three sliding panels that pull straight out and hold Thompson’s on both sides?
Someone else’s plans are just their version of what they think needs to be in their home, or someone else’s. That means the beast could be three feet wide, or six. It could stack, or be a single, solid unit.
When most of us started out, we felt we had to be lead down the path. Later, we could look at something and could guess at the joints needed to pull it off. We came to know they were not nearly as magical as some might suggest or think.
Years ago, I designed one for my rent-a-kid-brother. It was just a simple box that mounted on a wall and held a single 30-30 Winchester lever action. The front was glass I etched with an elk stepping over a log, with forest background, and relative Winchester info.
Will Google plans like this do anything for you?