DIY pond filter??

I have a 1600 gallon pond filtered by a diy bio filter. The trouble is the lack of mechanical filtration. The pump is a 1/10 HP so flow is good but the biofilter gets filthy so fast. I was thinking of changing the waterfall filter and addining a mech filter to limit the amount of debrie that gets in the filter. My plan is for 100 % of the water to go through the mech filter and 25% of the water to go through the biofilter the remaining 75% will go out the waterfall. So looking for ideas on a mech filter DIY preffered. Any ideas??

thanks in advance
Smaug