When I kept crocodiles I did not worry about having a filter. It was far easier and much more productive to keep them in a kids swimming pool. I emptied it daily and refilled it each day. I took it all down and rinsed it out with a hose once a week, including all decorations. I grew 4 of them from hatchlings to 4 footers before trading them to the zoo for some tortoises I had my eye on.
As far as making your own filter it should be simple enough but it will clog itself often. I cannot think of a way for it not to unless you were using 55g barrels. There just is not enough space, bacteria, or time for it to break all the way down. I now have 2 Eheim 2260 filters and 4 Eheim 2217 canister filters on my 125g pleco tank. I keep about 14 good sized plecos in that tank and every month all of the filters are clogged. The 2260's are 5g canisters. This means I have nearly 15g of filters on a fish tank that gets clogged up.
The simple way to go about making a filter is to get a good quality top locking 5g bucket. These have retainer rings that make it impossible for the lid to open or leak while the ring is down tight. Look for them in restaurant supplies. The locking bands are made from metal or plastic, the lids will have a huge O-ring in them. Get a bulkhead (3/4" or 1") and place it in the side of the bucket as close to the bottom as possible. use "eggcrate" from a lighting supply (about $8 for a 2'x4' piece 1/2" thick) and cut it to fill the bottom of the bucket. Cut about 3 or 4 of them to raise your media above the inlet of the bulkhead, this will help prevent blockage and allow the water to evenly distribute over the media. In your case I'd add about 3-4g of "bio-balls". These are common for salt water tanks as a wet/dry media. The price can be very cheap from sources like Ebay. These will trap larger particles and allow water to still flow through. They also have huge surface areas that may allow bacteria to grow and help neutralize your parameters. Then I'd buy some of those heavy duty floor sanding sponges that they make for floor waxers. They come in several grades and are easy to cut and stack to help filter the water. They do have a hole in the middle but it's easy to put a piece of PVC pipe with a PVC cap into the middle to force the water through the filter material. On the lid of the filter place another bulkhead. Plumb that to a good return pump like an Eheim 1262 and pipe that to the tank. By creating a vacuum on the return side water will fill the filter and actually level out with the back of the tank. The pump will barely work lifting the water the extra few inches over the tank.