If you've got the electronics and programming ability, yes. You could do it and save some money. Not necessarily. At some point you have to decide would you rather A) build an airplane with some minor personalization and end up with an actual airplane that flies. or B) design and build subsystems forever on an airplane that never leaves the shop. You just have to decide where that line is. I spent about 15 hours designing and building my own annunicator panel. Total cost was about $25. I could have easily achieved the exact same results with 1 hour of time and $10 in lamps. I decided it was worth it. In the case of an EFIS, that's a rather large undertaking... at least for me.