Your best bet is to do what Stevo67 said and to get a spare set of door cards cut the edge out so you have all the clips and clip holders to fasten it to the door. Then make a blank door card from M.d.f (NOT PLYWOOD) as ply wood expands with hot and cold and will crack and flex over time.
Fasten the plastic door card clips to the M.d.f using self tappers and something called kitty hair or
http://www.refinishsystems.com/produ...Filler-No4-Kit this will not crack when the door is slammed and gives a good surface for filling. You can also use this for smoothing the M.d.f into the plastic door card.
If you have never done anything using fibre glass and filler and resins my advice to you would be to try making something small to start using a speaker grill and mdf and glass to make a set of speaker pods that will give you the skill set to be able to do a door card. If you fuck it up it doesn't matter as its only a speaker grill and there cheap as chips. Modifying a door card as its a skill in its self too much hardener and the filler will bend the wood and make the glass crack. Too little hardener and it will never go off.
Any questions please ask as I have done hundreds of door cards and boot builds and pretty much used everything and seen everything been used in them.
As for getting them to match do both cards at the same time and make a skeleton using mdf and wood glue the stretch fleece material over then apply a strong resin over the top to give you a hard surface over the top to apply glass filler then easy sand filler to. This method means both will be the same or so close you wouldn't realise unless they were next to each other.
I still find it easier to modify the standard door cards than building my own as they weigh less and look more factory and less chav.
sorry for the million word essay.