how to do it?

[89chevy]

its a 1999 nbs extended cab. I am installing a new head unit, sub, and 4-channel amp, and 6.5 door speakers, ok, The head unit only has 2 rca outputs, right and left. The amp is 4 channel but has no high or low pass filters. what i am doing is running rca cables from the 2 outputs on the radio to the amp. Then im running the front 2 channels of the amp to the 6.5 door speakers and bridging the rear 2 channels of the amp to the sub. easy deal, but, what can i do that is simple and cheap to filter the lows out for the door speakers and filter the highs out for the sub? wish the amp im using was like my other 4 channel that had switches for filters built into it but it doesnt. want to do it cheaply and simply without any more power and ground wires than i have to.

will these work? http://www.ebay.com/itm/Bass-Blockers-Crossovers-6-5-6-75-6x9-Speakers-/220911302435?pt=Car_Speakers&vxp=mtr&hash=item336f56f723

thanks