Where is the storage going to be, in the rear seat bottom? If so, you need a solid base that the seat bottom can sit on. The seat bottom also needs to be strong enough to sit on without any further support, so the seat bottom may have to be beefed up or completely replaced. Once you have a solid base, the seat bottom can be done one of three ways. It can be hinged in the back, hinged in the front, or left loose. Most seat bottoms slide under the seat back, so the seat bottom may need to be shortened to account for this.
You are probably going to need to take it to an upholsterer, 'cuz there are more pitfalls to this job that you won't probably think of ahead of time. The problem is that this is a big job, and an upholsterer may not be able to do this as economically as you'd like.
No one lives forever, the trick is creating something that will.