From previous research and discussions here I recall that EGA (or was it CGA?) is near-enough to the boxMac video that existing E/CGA-to-VGA adapters work, with a little intervening circuit for polarity correction or something. Those existing adapters are not exactly cheap, though, and a pixel-doubled adapter to XGA may just work out cheaper, even in small/single/DIY runs - especially if you incorporate the aforementioned required signal conditioning onboard.

I noticed that you used the pixel-doubled approach in your FPGA Plus, BMOW - is there stuff you can re-use from that build?