How can I improve the stability of this DIY pull-up bar?
Added 2 eyelets per corner and used rope to tighten the tension.  Not quite an A-frame like the other answer, but it's now very stable.  I also noticed the instability was due to the wooden planks not being completely flat, so I padded the gap with magazines and doorstoppers, and now it doesn't budge at all.

Thanks for all the suggestions!