it's most definitely possible, especially if you have any sort of wood shop skills and access to tools.
although, in your case i'd hold off on building anything just yet. i'm assuming that since you can't afford a flight case then you're funding your own tour and doing it out of the back of a van. how expansive is the tour going to be? just your local state / province? tri-state area? up and down the coast? cross country?
first thing i'd do before cutting a single piece of plywood would be to find out what size vehicle you're going to be touring with. every case is going to add significant bulk to the gear and cut into available cargo space, especially if you're using a cube van / u-haul trailer (don't tour in a u-haul truck
). if your band happens to be lucky and features a van owning member then maybe you could custom build something specific to that particular vehicle. maybe even some sort of road case that houses the entire bands gear.
while it's definitely a great idea to want to protect your gear on the road the cargo space WILL be the determining factor into what follows you on tour.
another option if the plywood cases don't work out would be vinyl bags similar to the soft guitar backpacks a lot of people use. they'd be cheaper than flight cases, still offer full cosmetic protection and some kinetic protection, as well as being soft enough to reshape a bit if necessary so that you can squeeze in that last piece of gear / shut the door.