I made one like that and would suggest using 1/2" to 5/8" thread rod. The 1/4" will bow like crazy and score the threads when trying to get enough compresion and the thumb screw (if this is a wing nut) will most likly not work, you will be happier with a nut and use a wrench to get the leverage needed to get the compersion.
you will be supirsed at how heavy it will be when put together.
One more thing, i wished i had used a 2X6, with the 2X4 as you start to compress, the center portion of the taget material will try to bow out, it is hard to keep it centered during the compresion process and during shooting the force of our arrows hitting the target will push the target material out of center and pop out like a deck of cards. A 2X6 just might help this.
No need to make the target materal much wider that the board used to compress it as the excess will not comrpess as much and just makes to harder to get things centered.
I ended up changing from cardboard to carpet it lasted much longer but made the target even heavier.
My target face was 24" X 24" so you may not have as much trouble getting things centered and keeping it their.