It's easy to work out what weight you need, because it's the weight of 40 feet of water. The pressure at a depth of 40 feet is simply due to the weight of the 40 feet of water above.
Let's work in SI units, so 40 feet is 12.2 metres. Suppose the top of your tank has an area of one square metre, then the amount of water above it would be 12.2 cubic metres and this weighs 12.2 tonnes. That's a lot of weight!
The pressure is just 12.2 tonnes per square metre, which is about 1.2 atmospheres or about 18 psi. I'd be inclined to use a sealed tank and use a compressor to pressurise the headspace.
A footnote:
Several comments have warned about the dangers of pressurising a tank to 18 psi. I know nothing about working with pressurised equpment, never having had to do it, so you should not take my answer as licence to start pressurising old oil drums or whatever stuff you have lying around. Proceed with extreme caution and don't sue me if you blow yourself up!