I have been making my own stretchers, technically strainers since they cannot be adjusted with keys. They're stronger than standard bars at the store (since I use thicker wood and cross-brace them) and cheaper than the heavy-duty bars.
My question is, how much do paintings tend to sag over long periods of time? I'm wondering whether the cost savings will cause too much trouble 10(?) years down the line if I need to take pictures off their strainers and re-stretch them.