I meant, that if you institute stopping blocks.. the impact forces upon them can be quite powerful. The power will seek out the weakest link (breakage).. and that could very well be the choice of using particle board -vs- solid board. Also, the joints will also be stressed too... so if the construction isnt really strong.. a whole side might break off the bottom plate, for example.
(My initial device was a mere small machine screw that bumped into a rubber padded wood block set that Id glued in place. The forces eventually broke the block off. Hence, my advice to use something like a metal bolt set with bumpers - that goes through the actual shell)
Rubber bumpers will help, but even then a person can generate quite a bit of force in a quick-twitch flick of the wrists, in the heat of battle.
Re-enforce all the corners with blocks, much like the ones you have in the rear.
Additionally, it seems you simply glued the handles to the metal tube. This may also stress-break at end of travel. To prevent this, I had made two donut shaped collars that were cut at the correct handle angles. I screwed glued and screwed them to the inner side walls of the handles, using small screws. Then, the tube was inserted into the collars... and I put a long thin bolt through the collar top, and into the steel tube. This locked the tube to the handle quite well.. yet could be taken apart if needed.
Another option, is to cut the ends of the tube a bit, and make it so that you can bend two flaps 90 degrees, and screw them in place.
Yet another 'not so great' but dooable option, could be to tap threads into the tube ends, and lock it down with an internal & external nut. I dont recommend the typical use of threaded rod, as the threads anywhere else would shred stuff.. such as the bushings / etc.
The spring return could use some tweaking too. A quick helper may be to mount each spring end to opposite diagonals... to give more distance. They should be stretched out a little to create tension. It looks like if you turn one direction, the other spring will sort of bind up currently.
Wood is fine for a decent yoke. Just have to make sure its well built to take the abuses.