Hydra HV mezzanine board ideas

I've got an idea for a helpful item for building high-powered Hydras.

The bad news is that it's a little pricey.

Here's the concept:

This board combines the 12 VDC power supply, 2 contactor adapters and stuck/open relay detectors for two cars onto what I call a "mezzanine" board. The board is slightly larger than the Hydra logic board, but there will be holes strategically placed so that you can mount the Hydra logic board on 1.5" standoffs right above this board. One side of the board is all high-voltage stuff, the other side (segregated by the optoisolators and by the power supply module) is low voltage - the intent is to keep those two portions as isolated from each other as possible.

The board includes separate fuses for each contactor adapter circuit, plus a slow-blow fuse for the power supply.

The relay test circuitry is designed to allow the Hydra to detect and complain about relays that are stuck open or closed - that is, if the state of AC power to the car does not agree with the state of the relay output from the logic board, then that's a serious error that requires immediate attention. This version of the relay test isn't as comprehensive as OpenEVSE, but it can't be because the GFI coil for the Hydra is located prior to this circuit, so attempting to use ground as a return will trigger a ground fault. It also can't be as comprehensive as OpenEVSE because there's only one relay per car for the Hydra.

Unfortunately, just the board by itself from OSH park is $21 because it's so large - and that's my price. The power supply module is $20... This board will wind up costing almost as much as the Hydra logic board if I sell it in the store.

Still not sure what to do about that.