Be very careful with this much welding on a piston. The welding itself can be very difficult because cast aluminium items, particular of this vintage, can be a bit porous and years of sitting in oil tends to fill the pores with oil. Then no matter how well you clean them externally, they start weeping oil as you start to weld and then that's the end of it. Even with the right rods the piston will be softened unless heat treated after welding and heat treating can be difficult unless you know the alloy that was used originally.
I tend to think that your best bet is to look for new pistons that are close to what you want. There are a few piston makers in the UK, Omega for one who make specials. There are a whole stack of them in the US, probably a few in Germany too.
Have you tried asking around on classic bike forums? Try asking on this one
https://www.facebook.com/groups/367147517553916/
it has nearly 50,000 members so maybe there is someone there who can help. Maybe there are groups specialised in Douglas?
PS. machining businesses with CNC machines often charge what may seem to be exorbinent prices but they have to often break down whatever they are set up for, then maybe make some tooling to hold a special job and then put their machine back to what it was. However, there are many Chinese companies that offer very low volume CNC machining at very good prices even with shipping. I have friends who have used these and the quality is good. I know that there is an Italian company that does the same I will try to find their details. There are two alloy grades that are used for billet pistons and I will try to remember to dig that out also.
PPS. Another option. Rather than welding the pistons I think a better option would be to machine the grooves a touch wider and get some rings made. I have seen adverts for making rings from centrifucally cast cylinder liners. Which is probably what Douglas did originally.
Here is a tip for holding pistons for machining in a lathe if you decide to machine what you have.
https://www.homemadetools.net/forum/work-holding-lathe-90665#post201593