Reply to SallyK
This device is only a piece of metal cut to a round shape (a bit like the pastry cutters used for making minced pies!) There should be no reason why they could not be simply bent to suit whatever shape you want them to be. If you don't think you can do this for yourself, then maybe the manufacturers (nu-hope in this case) would do it for you. In which case you would need to provide them with a precise template for the shape you require. If they won't make you one, then almost any blacksmith would have the skills and equipment to do the job for you. Alternatively, you could look on the pastry cutters to see who makes those, then write a 'begging letter' explaining you problem and asking if they could do a one-off for you.
It's an ideal one-off job for an apprentice engineer or the local college or university department of engineering.
Over here in the UK , there is a programme on the TV. called 'The Repair Shop', also in our town we have a voluntary workshop where people 'mend' things for nothing. These sorts of places would have the skills and machinery to botch one up for you.
It seems to me that the problem is an easy one to fix - you just have to find the right people to help you.
Never be afraid to ask as there are plenty of people out there who would be only to willing to try to be helpful.
I use a craft knife for cutting the two sided sticky tape that I use for gluing my baseplates on. However, I use the baseplate as a template/guide because cutting a circular hole with a knife on a flat surface is not a simple task and it tends to leave sharp edges.
If you make a template of the shape you want, then there seems to be no reason why you could not just put the wafer on the template and cut around the hole. The template is the key to a good job and if you take the outline to a sheet metal worker, the job for them would be easy-peasy!.
PS: I have just come back from our weekly grocery shopping trip and whilst in the store I spotted a cheap stainless steel wine measure, which would make an ideal cutter if the rim edge was sharpened.