Funny your should ask...I'm just finished building a cage. But for my ST-4 Escort.
The number one best way is to show up at one of our events and bring a camera, tape measure and take notes. That's what I'd really recommend.
Anyway, for my Spridget, I'm not that crazy about the own cage since it's sort of cobbled together from an existing roll bar.It's safe, but sort of clunky.
But I also drive a Sprite (#23) owned by my friend Pete, and I like his cage. He built it a few years ago. It should be at Pocono on Sept 8.
Keep in mind that you must use DOM tube (not ERW) and for a Spridget it should be 1-1/2 inches nominal OD. You can get away with 0.090" wall, but I'd use 0.125" because we're all just amateurs and have to get to our regular jobs on Monday morning....a little more protection can't hurt.
Mounting to chassis can be bolted (5/16" bolts that are Grade 5 minimum). Mounting plates should be 3/16" thick. You can also weld it in, but it can be a challenge to weld thin, unibody stamped steel to 3/16" plate. Most of the amateur jobs I see don't look great.....I'd prefer bolt-in unless your welder is a real artist.
Pete's car has a horizontal bar that goes from the right side halo bar to the middle of the dash. He has a switch panel mounted on it. I'd skip that part. He also has bars to the front shock mount area that stiffens the car...not a bad idea. My car is a rubber bumper chassis and has similar stamped steel members in that area.
Sitting in the car normally, you need 2" above the helmet for the overall cage. The rear part of Pete's cage is high. Pete and I don't need the extra height, but he built it with our pal Bruce in mind (Bruce is like 6'3").
Be sure to have a good shoulder belt mount. It's best exactly at shoulder lever and not too far back behind the seat (2" to 4" is good if you can do it). Generally, this is a horizontal bar that the shoulder belts loop around, but you can also weld plates to bolt the harness mounts to it.
Also, try to keep in mind an upper seat mount (for the seat back), since that's required for most seats.
A horizontal bar behind the dash connecting the two front down tubes is a good idea too.
You can bolt or weld the door shut but if you do, it can be a pain fixing thngs. #23 has it's oil filter and alternator in the right side passenger area, so the doors open and close for easy service.
And check the EMRA Car Prep guide (similar to SCCA).
https://www.emraracing.org/EMRARULE.HTM
Some of the other guys may want to chime in...that lots of ways to do this.
Here's pictures of Pete's car....this is all I have on hand.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
