Obviously an old bell housing is best solution.
I can't remember,? Did your Panda have a tin plate sandwiched between gearbox and engine, this would help in making a adaptor mount from thick steel.
Even without that it should be possible to fit some flat steel from bell housing mount bolts, picking up a couple of positions either side to make a robust fixing with a little trial and error to position correctly.
Quite a bit more involved, but I managed to make an adaptor to fit a Ducato 2.8 turbo engine onto a Volvo Penta Sterndrive in place of the original 150Hp V6 petrol engine on my boat from scratch using 5/8th steel plate, alignment had to be pretty good as it had to pick up the drive plate bolted to the flywheel that the Volvo shaft engaged in, I ran the boat for four years with no issue.
Apart from cutting the steel using a Oxy/Propane torch, most of that I did in a lock up garage with basic tools after careful marking, just using a drill and angle grinder.
If you are worried about damaging the starter and ring gear if not perfectly aligned, a little trial and error with spark plugs out and just a quick on and off of the starter operation doesn't generally cause much damage, you will soon hear if it isn't happy.
If it is too deep in the teeth it probably won't allow the pinion to push forward into the drive gear, if out too far it will just spin off the edge of the gear, so as long as you stop immediately then unlikely to cause any serious harm. I have seen many cars with the starter bolts loose and the starter all over the place making strange noises and on inspection and refitting securely no further damage was caused. Using common sense of course
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Could you use the bell housing on your original gearbox as a template using cardboard to mark it out and transfer that to some steel?
