I made a very cheap 20gal refugium. Here is a list of the things I bought for it
20gal tank with hood - $20 (it was used)
Overflow box - $20
valve - $10
tubing - $15
175gph powerhead $19
CLAMPS!!! $8
my calupra (sp) and live sand were donated

Your flow rate will be slow. if you want more flow, just upgrade the powerhead. My goal was to allow pods to survive the trip up to the main tank, and to allow the calupra to remove nitrates.
The overflow is in the main tank, the valve is on the line from the overflow to the ref. The powerhead is in the ref and pumps water directly into the main tank. Make sure your tubing going back into the main tank empties above the water line, if you have it underwater, a poweroutage will cause it to siphon back into your ref causing it to overflow.
once you have the powerhead running, use the valve to match the volume of water entering the main tank.
I just hooked it up, I'll let you know how it works out