I had one of those dolphino easy-set pools ( 18' x 48") & I put a 20' x 4' solar panel to it. I had aprox 50' of hose ( 25' each way) & I'm not sure how many actual feet the water had to run through ( through the panel vanes). The hose that I used was about 2" diameter & so I had to use flexible pipe conectors with O-ring attachments to make it work.
The pump did not seem to have any problems handling the extra work load.

This DIY method that your speaking of could work... check out some free plans on the internet though before purchasing anything through Ebay. Generally, you can come up with some great ideas on your own... making a solar collector is very easy... ( ever leave your garden hose in the sun? water in it gets hot pretty quickly) .. I've heard of some plans for modifying old satallite dishes for this purpose as well.

Btw, my solar panel ( which I purchase at a close out sale from a store), would heat up the pool by about 12 degrees on really hot days

Good Luck!