One side of my flightsim table top rests on my home-build subwoofer. It measures 750mm high x 600mm deep x 260mm wide. Material is 20mm particle board. The speaker is a 8 inch woofer that I bought in a Taiwan electronics shop for USD 30,-.
From acoustics point of view, it makes sense to make the box big. Speaker characteristics together with inner box volume determine the low frequency roll-off point. Too small box volume will limit the low frequency range. For nice deep engine rumble, low frequencies down to 30Hz give great realism. Most multimedia speakers roll-off around 100 120 Hz, and the so called "Subwoofers" sold with small PC audio systems may go down to 70Hz or so (if you are lucky), adding lots of distortion along the way. Keep in mind that the difference between 30Hz and 70 Hz is not trivial.
A reasonable quality 6" - 8" speaker in a large (100 liter or more), heavy box with 15W power input will outperform any MM subwoofer, unless they are of comparable size.
When buying subwoofer speakers, it is hard to get any speaker spec except for power and impedance. If you are able to find more spec details, check for low self resonance frequency (around 25Hz) and low Qts (around 0.45). Power handling is normally not a problem, as 20W (RMS) already gives plenty sound from a large speaker + box combination and will match the sound from the L/R speakers.
My box reaches 35Hz, and the sound pressure in the room makes the windows rattle. I used a completely sealed (really airtight) box construction.