Most people use whitworth allen caps and screw them through the new sump into the alloy. I think this is a bit risky as the internal threads in the crankcases are short and prone to stripping anyway. Unless you are very careful making sure your threads are good, and you use the full length of the thread, the potential for stripping is quite significant.
It is funny how long a thread about a thread can get Richard! But I agree 200% with b'macc on the use of studs not cap head screws (as supplied by, eg, SRM for the plate I bought). These are about the most strip-prone threads on the bike, and are often worn to start with.
Surely the best way of doing this and a million other such things is to obtain some basic threading kit and bits of decent steel stock in the usual imperial sizes. For those of us stranded in parts of the world where Brit imperial stuff is unobtainium , it is pretty well essential. The kit is easily available, the initial outlay is a few (not that many) beer vouchers, but thereafter . . . empowerment!
The low-cost way to keep a shedful of stuff up together is to get the right tools, and then get a small lathe (and some inexpensive supporting literature). Total outlay less than many an engine build, benefits last for ever, satisfaction immense - and the learning curve is pure pleasure.
I divide my shed life into 2 parts - the Days of Make-Do, and the Age of Empowerment.