I really struggled over the decision to put an access in the back of the baggage compartment - I really wanted to make maintenance easy, but was not really sure I could make a nice "hatch" - and also realized that I'd still be working through a small hole if I did.
What I finally decided was based on my decision to go with a full EFIS panel - the GRT EFIS Display Unit is 7" wide, which means I ended up with a hole that wide and the height of the panel, right in the center - a pretty good access, simply by pulling the Display Units. I finally decided not to cut an access when I realized that in wiring up the panel on the bench, I had built in disconnects for every line, so that I could install and remove the panel in about ten minutes (I've actually pulled it into my lap in five - it takes another five to leverage it out of the cockpit...).
I'm not saying that an access panel isn't a good idea, I'm just saying that there might be alternative solutions if you look at your entire system. The position of my EFIS AHRS box would block most access hatches. Just lay out your systems, and then decide how to do it!
Good Luck,
paul