I made a set out of 2x8 pressure-treated lumber. For each one I used (iirc) a 36", a 24", and a 12" piece (6' total for ea). U can use lag bolts (countersunk), or carriage bolts to put em together. Total cost was about 25 bux or so for the pair.
I originally made these to make chaining up my big truck easier, just place one behind or in front of the inner tire and drive on. Outside tire goes up in the air, chains go on so tight I didn't even use bungees. I only throw that last part in to say, yeah, they're plenty strong.
You could make em taller, these were high enuff for what was thier original purpose, about 4.5". I use em for oil changes and to gain a little height for floor-jack clearance cuz the front of my car is too low for my old school, high profile jack.