To avoid a lot of hand-wiring, I bought an 8-bit ISA bus board from eBay. This reduces the effort to interfacing an STEbus slave prototyping card to an 8-bit ISA bus connector.

There is some TTL and a PAL16L8 doing some address decoding logic.
The RS232 buffers are 75185 chips. 
The UART crystal is 3.6864 MHz, and is halved to drive the 16C552. This limits the maximum baud rate to 115200 baud.

I could solder wires directly to the edge connector, but fitting an ISA bus socket would allow me to experiment with other ISA bus cards without soldering and desoldering. So I bought some sockets.

Most of the signals are address, data and power lines. The remaining signals are 8088 style, and it should not be hard to create them from the STEbus signals.