hello everyone
The Gameboy cab was made by hacking a GBA Advance and connecting those to the buttons (pic included). A 13" TV was used as the monitor and a GBA TV adapter (bought on eBay) was used to send the video signal to the TV. The onscreen picture was slightly smaller than the TV screen but it looked pretty good. The GBA adapter also had sound output which was connected to the TV. The hard part was soldering the wires on the GBA because the terminals were very small. Once soldered I had to put some hot glue on them so they wouldn't move. Here's a pic of the GBA soldering points.
I just checked eBay under gba tv and there's like 4 of these up for bids...the bottom pic is from one of those auctions.