Let's build a variable air capacitor the way it was done before they became commercially available.
First step determined the size you need. Just to make it simple let's say the plates will be 2" X 3" and we will do 5 plates. The unit will be piston style. with 3" plates we need a base a little over 6". A piece of 1" X 2" 13 or 14" long. Layout the center line length wise and then make two parallel mark 1/8" either side of it. What we need to do is make sallow grooves on the marks. I use a handsaw and an awl. Use a straight edge to scribe the lines. The scribe line just needs to be deep enough to guide the saw. Take the saw and follow the scribe lines to make cuts about 1/8" deep. You can use any convient metal for the plates. I prefer steel flashing.Aluminum will work but it is harder to solder. Once the metal is obtained check that the grooves will allow it to glide freely. If the grooves ar satifactory cut the board into two equal length pieces. Mark the orientation so you will keep the same edge to one side. If the grooves are not purfect they will still match. Place three plates in the first, middle and last slot of one board. Cap the unit with the other board. You will need to secure the assembly. Take the other two plates and slide them into the remaining slots. Solder a piece of wire or some scrape plate material to the two sets of plates. attach a handle and hope it looks like this. (give or take a little)
Once you have the idea in your head you can add more plates and make whatever size you need. With a little imagination you could close in the front and add a screw adjustment. You could even use a split design and have a large section and a smaller one for main and bandspread tuning.