I have two concerns with your design that you posted on page one.
1. Tubing will crush if bolted down tightly, and of course you want it as tight as possible so it stays together. You can solve this by cutting smaller tubing that the ID is a hair larger than the diameter of the bolts you will be using, length cut to the ID of the larger tube. Square tubing for the chassis would make this easier.
2. In most places, you have one bolt per joint. If the bolt loosens, the part can swivel so you'd want to make or buy brackets that will go "over" each joint and have at least two bolts per tubing where they joint. For example, if you are joining two tubes together in a "T" fashion, you'd want to make a "T" bracket to go over them. Two bolts in the cross piece of the "T" and two bolts on the one leg of the "T".
Of course you could buy a used welder on the cheap, even a larger 120V machine, and learn to weld. Wire-welders are often reasonably priced and are fairly easy to learn. If you choose this option there are many here who can give you advice and offer guidance if you wish.