You can make some panels out of construction cardboard. You can get this from any upholstery shop, or supplier. I pay 2 bucks a sheet. 40"X60".
Cut some pieces as large as possible to fit flat to the door panel areas that you want to cover. The round areas can be done in several pieces. Then, by gluing several laminations, of the cardboard over top of your first layer, you'll end up with an 'insert' the shape of the lower panel. You don't have to make each of these laminatons in one piece. Start with small pieces, building up to larger ones as the cardboard begins to hold the shape of the lower door panel. You can fold 1" wide strips in half to tuck into the corners to attach the 'crap collector' sections to the flat sections, between layers. Weight down the cardboard, as the glue cures. The edges of each cardboard piece have to be feathered out to the last layer with a mini-angle grinder & a medium-coarse sanding disc. 80-120 grit is what I use. You can't use too much glue between the layers. Keep building up the layers until you have a piece that is about a 1/4" thick. Leave about 1/8-3/16" around the edge of your new panel for fabric clearance.
Now you you have an insert that can be upholstered. I'd put down 1/8" foam, then cover that with whatever your using on the upper insert.
To attach the finished panel to the door, I use 'PL-400', a commecial construction adhesive that is available at any home improvement store.
This is time consuming, and requires wait times between gluing the layers, in order for the glue to dry, before sticking the laminations together.
I've built complete under-dash covers in Hot-Rods using this method, door-door, dash-floor, in very intricate shapes. Some of these have been 3/8" thick. As tough as any plastic panel.