Portal shaped like the ender portal

You’re going to have to make a procedure to test for blocks in those specific areas, then you’re going to have to make a portal texture, make a block with that texture, then in the procedure, make it so when all the blocks have been tested for, to place the portal blocks in the inside area. Then of course you’re going to have to make another procedure, so that when you go into the portal block, (you need to be able to walk through it) it teleports you to the other dimension. Sorry for the long reply :)