Companies receiving these contracts (either fixed-price or cost-plus) will be incentivized to maximize profit and the expense of the shareholder. I don't think it's possible to remove this incentive. Instead, the government should be more careful with whom receives the contracts. For instance, SpaceX seems dedicated to doing a good job and driving costs down. Therefore, right now, SpaceX should receive more contracts. SpaceX's focus may change in the future, and it's the government's responsibility to pay attention to those changes and change contractors if SpaceX starts leeching off of government funds. Adequate competition for contracts will align incentives more than the structure of any particular contract. Everyone here saying that Lockheed turns a 10B problem into 20B of work due to cost-plus might be correct. But why can't the government find someone to do the work for 10B? With proper competition, incentives will align.
