You'll need to add weight to the trap proper and weight to the doors. Rebar is cheap and easy to install. Tillamook Sporting Goods sells used floats when available. You won't save much but if that's what you're looking for......they sometimes have them.
One thought I have when I look at this picture is that you could potentially be creating an electrolysis problem by simply throwing rebar on your traps. When you have dissimular metals in salt water, it creates a battery so to speak and depending on what metals are involved, could make your pots hot.
Check out a galvanic corosion chart to see where the different metals stack up. Zinc is on the anode side while gold and platinum are on the cathode end with everything else we use for crabbing/fishing in the middle.
I have heard both school of thoughts state that "hot" pots do not perfrom as well and then it doesn't matter at all. Regardless of the crabs sensitivity to electrolysis, it will ruin your pots over time. Now with these cheaper, light weight pots, it may not matter as they can be replaced easier than adding a zinc anode. However, if the crab are sensitive than you wouldn't want this phenomenon happening in either case.
Personally, I have experience though the years of certain pots consistantly performing well and ones that do not. I started noticing that the ones that perform poorly had more rust and the rubber coating had broken down quite a bit over the years. This would expose the steel frame to the stainless steel cage wire creating my electrolysis.
I am by no means an expert but if you simply coated the rebar with rubber tape before adding it to your pots, it would help to mimimize any electrolysis.