This module actually generates a "Decay only" envelope. The trimpot controls the amount of time it takes for the output voltage to decay from +5V peak value to zero. The maximum time is determined by the capacitor. I'm using 100 uF here but if you want a longer decay you can replace this with a larger value. Alternatively, you could add a toggle switch to select different capacitors / decay time ranges.
My total build cost was less than 50 cents because I only paid for the perfboard and header pins. The other parts were all salvaged from trashed electronics but even if you have to buy the whole lot, it shouldn't cost more than a buck or two.
Panelizing this module would only require two (mini) jack ports and a panel-mounted potmeter plus a pinhole for the LED and a range switch if you added more capacitors.