So much depends on the type of robot you want to build the available skills, money and time.

Saint has a welded Hardox equivalent drum with conveyor belt wrapped round that, the top is 20mm UHMWPE or RG1000 nothing has ever got close to penetrating either.

Cherub has a welded lid and sides so the bodywork for the robot comes off in two bits, the back panel and the rest. this is welded 5mm steel again nothing has yet penetrated it.

I don't like bits just bolted on as they usually weigh more and protect less. but this is what I did with Seraph using only 4mm studs and screws. and only 8mm HDPE for protection. works well for that type of robot.

So in conclusion it's
Horses for courses.