OK, the single plexi is available from Vintage for $600 or so.
As Carlos says, switching back and forth with the Speedster glass is a little tricky, but I have found that winches and suction cups are not needed. It's done like this:
First, install the glass windscreen completely and perfectly. The tricky part is the end frame pieces, which leave about a half-inch oval home in each side. Your builder will do this as a stock, no-extra-cost thing.
To remove the glass you loosen the bolts on each end, and lift the glass and rubber from the thin aluminum channel which will have been riveted to your cowl.
To make the whole thing convertible you have to carefully drill out every rivet and replace them with 6-32 nutcerts (AKA RivNuts). Then use small head stainless screws to install your screens.
(Don't worry, on yours they'll be more evenly spaced).
You'll want to remove the wipers and maybe cap the shafts as well.
Then you install the little racing screen and try to get at least one or two of those holes lined up with your existing nutcerts. Drill and riv-nut those as well.
If you paint the riv-nuts body color they'll become less prominent. In the photo above you see riv-nuts for the glass channel and the full-screen plexi screen, and with either windshield installed they're all hidden under weatherstripping.
—Which reminds me: a lot of Speedster windshields are installed with a good bit of black silicone goop to fill any gaps under the aluminum channel. That's good for water-proofing but bad for convertibility. Use a bit of fender welting instead.
Finally, about the big holes where the wide screen bolts in: I sliced a bit of synthetic black wine cork to fill mine. It sits flush (and hides under the full-width plexi screen. You could also make plugs and paint them body color.
All done, you'll be able to swap windshields in about an hour. Not as quick or easy as you really want, but not impossible.
Oh and PS: I believe our own @Cory Drake has a plexi screen in a normal Speedster frame in his Hoopty. He may be able to advise on how practical that is to do.