Don't throw out your old trestles and instead turn them into height-adjustable hurdles to train your dog (or cat/guinea pig/etc.). A very quick build to help spent extra time with your pet.
Step 1: Supplies and Equipment
This build can be accomplished with a wide variety of tools, but since my budget is quite limited I had to work with whatever we had. Which was:
2. Small wood saw
3. Wood file
4. Ruler (90-degrees / right angle)
5. Regular, straight ruler (optional, but handy)
6. Large wood saw / jigsaw / table saw
7. One (or more) trestles
8. Thin (bamboo) sticks (optional, but recommended over thicker pieces of wood)
I used a jigsaw to make the longitudinal cuts, which was quick but rather inaccurate. I imagine a table saw to be more neat, but you might then run into troubles since you don't want to cut the wood along the full length.
The bamboo sticks I used were such that my dog wouldn't get hurt when she knocked one over, though a slightly thicker piece of wood might be preferred if your dog is not really familiar with hurdles yet.
Step 2: Measuring and Sawing
Take out the vertical pieces and cut them like in the second picture, so horizontally. Their length is fairly unimportant, as long as they're long enough to stably balance the bar and short enough to leave enough room in the middle. I cut them at about 10 cm length (about 4'').
Next, cut off 2 vertical pieces from the original stand, such that you end up with only one vertical piece per side. Please refer to the final pictures for further clarification, but basically you want to have clear access to the wing nut with which you attach the mounting points.
Finally, cut the remaining vertical pieces lengthwise, such that you end up with a longitudinal gap. Again, width is not very important as long as it does not exceed the width of your bolt and wing nut by too much. I made it about 1.5-2 cm wide (about 0.8'')
Side note: the wing nuts and bolts were already part of the trestles themselves, so I didn't have to buy them separately.
Step 3: Finalization and End-product
Finally, take the mounting points and attach them to the vertical stands (you might want to add a small ring on each bolt to increase the effective width of your wing nut).
The longitudinal gap makes the hurdles height-adjustable, simply by loosening the wing nut, moving the mounting point up or down and re-tightening the wing nut.
Side note: if you wish, you could paint/coat the final product to make it look nicer.
Then, put on the bar and get your dog ready! Also, keep in mind that hurdles are one-directional. Meaning that the bar can't fall of if your dog hits it from the wrong side.