Ill have to agree with the above comments. I am by all means not an expert but I do choose to ride with clip in pedals and would never compromise (or doubt) the integrity of a decent mtb Shoe. I myself, happen to use Crank Brother’s Candy pedals. (I got them on ebay for $35… usually they are $60-100, depending on their alloy)… for a while I used an old pair of road shoes b/c I refused to dish out the $80 or so on new mtb shoes which i was sure couldnt make that much of a difference. Man was I wrong… Ever since I bought a pair of Specialized shoes ive been in heaven. MTB shoes are designed in a way to give you the most efficient leg revolution while putting your put in the best possible position of comfort. (My shoes are built up on the inside of the shoe, which makes my pedal very direct and efficient). Besides that they are extremely stiff, which gives me alot of control over the bike in rough areas.
I would not recommend making you own shoes b/c I dont really see it working in the long run. The road shoes I used previously were not only extremely uncomfortable in rough terrain, they also got torn to pieces due to all the trail riding and torque put on them. I acutally glued the bottom sole back on a couple times, even using epoxy at one point, but it was to no avail… I cant see how you’d be able to make an old sneaker have the ability to really hold a clip in when it mattered. But then again… why not try it and see what you come up with, I just wouldnt trust it lasting for more than a run or two. Anyways, good luck, make sure you repost if you do come up with a good cheap idea, im interested in hearing about it!