The way I understand skin effect it has to do with surface area of the metal. Assume you have 1 solid wire 1/8" diameter and a bundle of 10 wires that are also 1/8" diameter for the bunble. There is several times more surface area in the bundle of 10 insulated wires. A 1/2" diameter copper pipe has the same skin effect as a 1/2" diameter solid wire. I do not know if electrons will travel through the center of the hollow copper pipe? It seems to me they should unless both ends of the copper pipe are soldered shut.
You can make your own Litz wire with a roll of enamel coated copper wire. Today my project is to wind 3 ferrite choke coils with 40 turns of Litz wire using 50 strans of #26 enamel coated copper wire. It is 2" around the toroids, to get 40 turns I need a wire length of 40 x 2 = 80" long plus about 4 extra inches. Solder the tip ends together then put a small amount of twist in the 50 wires this makes wires all the the same length around the toroid other wise some of the wires will wad up near the end. I might need to varnish all 50 wires to glue them together then let them dry before winding the toroids.
Those 100 foot tall mega watt 300,000 volt power lines that run cross country are 1.5" diameter solid plastic covered with a layer of aluminum foil over the outside.
I built a power supply for a rail gun instead of using wires to connect 48 capacitors together I used metal straps 1/16" thick, 2" wide, 14" long. Caps are assembled in a square, 4 caps wide, 4 caps long, = 16 caps per square, 3 layers of caps, all layers connected with metal straps to reduce skin effect. Caps are connected in groups to keep all the connections short as possible. Discharge from the cap bank is at the phycical center of the metal straps not the tip ends.
I built a Tesla Coil using 3 layers of #24 enamel coated copper wire in parallel side by side on the secondary coil 11" diameter PVC pipe. 3 layers of wire in parallel reduces skin effect, it also reduces wire resistance. The output of the TC increased 10%. 132" sparks turned into 145" sparks while every thing else remains the same. I posted this on the Tesla Coil forum about 10 years ago.