I purchased the Kreg table as well. I built my tabletop using 2x4 stringers as you mentioned I notched them, so they are flush with the Kreg U channel and the used 2 sheets of 3/4 MDF. I’ve been running my Journeyman on it since late September with no issues I can see. My waste board also 3/4 MDF with t- tracks and threaded inserts mount to my top so effectively 3 - 3/4" sheets of MDF. Put it on the Kreg casters and its very solid and moveable. Personally, I wouldn’t change anything. It has always trammed easily and holds the tram when I resurface my waste board. Hope this helps.
I am in the camp that you want the most advantage you can give yourself in terms of a table. Key is to assure that he Y rails are coplanar. That said, a torsion box or similar structure is what I would personally use.
I just used a few 1.25 " spax screws around the perimeter and when I mount my spoil board, I use 1.5" Spax screws recessed 3/8" into the spoil board so it catches all three pieces of mdf. Like I said this is the wat I’ve done it may not be the best, but I have had no Coplanar issues.
My 2 cents, anything using dimensional lumber is not going to stay flat enough with changes in humidity for a CNC table. I’d suggest using 2 layers of 3/4" plywood ripped to 3.5" laminated together in place of a 2x4 for long term stability otherwise you’ll be chasing an out of flat base every time the humidity levels change. You could also look into a laminated ‘stud’ type offering from a lumber yard, they are very dimensionally stable but a bit more costly than dimensional lumber.
I’m personally using a 8" deep torsion box and it’s remained stable and flat over over a year.
FWIW, I have had mine on 3/4 ply and 3/4 MDF glued together and sealed with Poly for nearly a year with no issues to date (in Texas). Toyed with doing a torsion box and also with runners (Similar to what Keith did), but just decided to get the QCW instead for future proofing, but like i said no issues with bowing or sagging even without the spanners over this time. Sits in my non-climate controlled garage…
My issue is that I’m in a temporary space with a not super large elevator. I’d love to not build too many more gigantic immobile things knowing that I’ll have to move them in a year. Two sheets of MDF screwed together for the bed kinda works.
I have the Kreg 44x64 bench with a sheet of 3/4 ply on top. I was going to build a torsion box but once I sat the QCW frame up on the table I saw that the feet of the QCW frame are almost directly over the legs at all four corners. So weight is transferred directly down to the concrete floor. The only wood in the support structure of my Journeyman is that 3/4" of ply. Everything else is steel or aluminum and concrete so I don’t think it is going anywhere. I’m adding a lower shelf to the Kreg bench on which I plan to add some ballast, at least 100 pounds. I also plan to enclose the sides with some 1/2" plywood (or may go with diagonal bracing) to add rigidity to the bench frame. But it doesn’t feel like it needs it. The bench is very rigid now. I did not put wheels on the legs which I believe helps with the rigidity. At this point I don’t see the table moving anywhere and have no plans to do anything more than described above.
I used 2x4 ‘s for a torsion box and sorry to say that it started out ok and then about 3 months later one of the 2x4’s decided to twist. I used a sawz all to cut it in the middle and it will wait till lumber drops in price to replace 2x4 s with 3/4” plywood