To keep it simple, I built a cylinder dipole for 21 MHz with two Monster energy drink cans of 500 ml and a coil. The cans are very good available, although I don’t drink the energy drink my self. The cans are made of aluminium and are easy to treat with a tin shear and an awl.
With an awl I pierced a hole in the bottom of the drink cans. Then I made radial cuts with a tin shear in the bottom of the cans and folded the points inwards. Please be careful for your hands. I made cuts from the drink hole in the top of the cans. I don’t remove the rims, so the cans keep their strength.
I connected the wires to the cans with a screw and a bolt to get a good connection with a low resistance.
Then I put a white PVC tube of 40 mm through the cans to support the cans. The cans can be shifted along the tube at all times.