Regular pine that you get at Home Depot. Screwed it together with drywall screws.
The only caveat on this is just to be smart about it. The A frame 2x4s are just touching the floor, and also just touching the bottom of the 2x6, which is flush with the shorty 2x4s holding the top of the A frame together.
What I'm getting at is try to get the most of the load going through the wood and not the screws.
I'm not 100% convinced this is a perfect design (I know nothing structural about wood), but I think it's fine for this job. There's not really that much weight on it after the forks and wheel come off.
It sat like this a few weeks while waiting for Thermosman to do the forks. No problems. I even worked on it and put on the rearsets while it was like this.