Nice work. Parallax make some great products.
If you use the FTDI chips for the serial to USB conversion you’ll get a much smaller device. You could fit this circuit in a much smaller space if you used SMT and a different MCU. It will probably run off a lithium button cell too then.
I’d say a Microchip PIC16F628A and one of the FTDI Serial-USB chips will have you sucking diesel :)! Of course you’ll have to use ASM or C to program the PIC if you remove the BASIC Stamp 2. But a BS2 is just a PIC programmed to run basic.
There are companies that will produce bulk orders of small electronics goods (eg runs of 1,000) but you’d want to be sure you can sell it first as it is an investment.
If you wanted one or two there is a Bulgarian company, Olimex, who make really nice prototype PCB boards for you if you send them the appropriate CAD files. There is loads of free software that can do this (like KiCAD).