Jim Kleiner created a minimal FPGA cape for the BeagleBone Black:
Jim describes his design decisions:
I decided to try a minimalist hand solderable FPGA board. The LX9 is the largest part available in a TQG-144, beyond that its BGAs
One of the key points is that the SPI interface is on BBB pins which are accessible via PRU0. This is important since the PRUs can be used to obtain higher performance SPI interfaces than the built in hardware units (examples here, here, and here).
The KiCad desgin files are available on GitHub:
The overall cost sourcing from OSHPark and Digikey is $12 for the PCB (quantity 3), LX9 $18 (quantity 1), connectors $3, oscillators, and regulators $3, Flash and passives $3 for a rough total of $39 per board.