Could you not buy a cheapy pair of hinges out the poundshop, some fibreglass filler out of halfords, apply it and then shape it to look correct mate?
I aint got a clue what the hinges look like but if they're particular ones that might be of some help!
Ian mate, I'm not familiar with the mortar shells but im assuming they're eletrically set off?
If so, I think I remember reading about someone using a 9v battery, so I've come up with this (sorry if it comes across as patronising, you've probably already come up with it yourself but you never know!)
The green bits are contacts to complete the circuit upon depressing the original firing button. I reckon with a bit of fibre glass and a dremel it might take you an afternoon or two at the very best?
I'm aiming to fire nerfs and such via m203's so it's a bit hard to get it right without the fugly rod thing sticking about the back that everyone is happy with. I'm thinking of cutting a trap door in the top to breach load it, then have a spring loaded firing pin behind the shell which is connected to the firing button. Its a simple enough theory but I just need to think of and make A) A spring loaded firing pin, don't fancy adding that to my search history a way of recocking without having to add the ability to open up the whole damn thing. Probably deceptively simple but I can't really get to the bottom of it. I've also toyed with the idea of using a 9v to power a small hobby actuator but I think the time between firing button pressed and that fully extending to press the back of the '203 could be between 3-5 seconds which well its a bit useless really.