my 1st reef was a small bowfront, around 30 gallons and it was undrilled and back then, a sump was a four letter word (still is i guess
), but I ran a rena xp1 that I drilled a hole into, ran my heater in and sliconed around the hole I had drilled and took all the media out and packed it with LiveRock rubble and voila, mini sump with just one hole drilled.
Worked great for me and the rubble was easy to rinse detritus out of when then need arose as it was all in the media basket.