You don't say what the make or model "contractor's saw" you have. I'm really afraid that if you have a low-end saw, that there is really no help for the wimpy fence that came with the saw. Adding to the existing fence really doesn't solve the problem as stated previously, more than likely it is a lock-up issue. However, that doesn't mean that some of the higher end saws in this category aren't good, some are excellent.

I had a bottom of the line Craftsman that was purchased to do laminate flooring installations. It was downright dangerous for any serious woodworking. It was nothing more than a circular saw mounted upside down. The fence was terrible and would not properly lock-up or hold settings. I could go on, but you get the idea. It was replaced by a higher end "contractor's saw". My replacement happens to be a Bosch 4100 which is an excellent saw in this group. The DeWalt is, also, a good saw, but I have had/seen DeWalt owners dump them after using the 4100.

A Bosch 4100 refurb. can be had for around $500 if you shop around. The DeWalts for a little less. This may not solve your problem, however, by the time you attempt to fabricate a useful fence you will be on your way to a good chunk for usable, safe and accurate tool.

I know this isn't what you want to hear, but face the reality...the fence on your saw is inadequate and that isn't going to change.