Then it depends on the trick you want to do. If you want to do the "stand up bass" trick ala Al Rappa of Bill Haley's comets you first need to reinforce the block where the end pin goes through. You might need to add a plate of wood to the bottom of the bass and bore a new hole for the end pin. Don't count on screws alone, use glue too.
You play the bass with the pin sucked all the way up so that the bass balances on the pin, (you might have an old rubber crutch tip on the pins end), and that area of the lower bout that touches the floor. Kinda like a tripod. Side bumpers help keep too much of the weight from pushing on the edges of the top and back tables. Every bass is different so you need to look at the vector inside the bass where you need to build a framework that takes the weight of your body from where you stand to the floor. 2x4s are not necessary, One by stock is pretty much all you need.
The insides of the ribs need to be reinforced too with plates of wood. The best jobs actually follow the curves inside the ribs. I'm not sure but I think Al stood with one foot on the inside of the C bout and another on the lower bout. (Up date: most pictures of Al show him standing on the upper bout of the bass) That's probably the toughest retrofit to do. Actually the toughest retrofit is changing minds...
My pal Robby likes to leap to the lower bout of his brother's bass while Tommy has the bass all the way down on its side and continues to slap while Robby plays that big orange Gretsch behind his head. The crowd loves this. Obviously, this retrofit is a bit easier to do as the line straight through the bass is simpler and does not come near the sound post. That's another problem... Putting that much weight on the ribs tends to push the top and back tables away from each other and the sound post can fall down.
Unless you know exactly how to do it, it's trial and error. You go through the side of a bass and then while the top or back is off, (whichever one was the most screwed up when you crashed), you can do all that interior work. It might actually take several tries to get it right, and if the audience likes the trick, they love it when the trick fails and the bass collapses. Man, the sound of a highly amplified bass goin' down that road is something... I've heard of some basses that were built so well and tank like that they never collapsed and never needed reinforcement! But if you need to do the deal, try to make it as light as possible while making it strong. I would be interested to hear Eric Swanson's take on this. As a cabinet maker and wood worker as well as a lover of all things bass, I would think he could contribute to this thread usefully. Come to think of it, Gary or Arnold would be fun to hear from too, but it might be tough for them to get behind this idea. Kinda like asking a doctor to take out your appendix before it actually needs to be. Do luthiers take a Hippocratic oath?
Again, I would say that anyone who wants to try these showy tricks should go for it. Bass playin' and performing should be fun. Certainly don't let anyone else tell you what to do and try not to talk yourself out of it either. Remember what Jonathan Livingston Seagull said: "argue for your own limitations and sure enough, they're yours"
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