This is the single most important component on your race car because it can literally save your life. It is also the item that causes the most teams to fail tech inspection. Because Lemons let you idiots race such old and shitty cars, their standards for roll cages have to be that much higher than most other series. An SCCA or NASA roll cage might only need minor tweaks but a drit trackl or drag racing cage will almost certainly fail.
Unless you are a professional welder, do not attempt to fabricate your own cage.
If you are thinking of buying a car that has an existing cage, send pictures to the Lemons Tech Guy, John Pagel (john@evilgeniusracing.com), to get his approval.
If you are a fantastic welder, you can get a prefab roll cage for many common shitboxes from Roll Cage Components.
If you are a decent welder and would like to level up to be good enough to fabricate your own cage, John Pagel holds classes once a year.