Anyone have a poor man's way of doing vertical knee raises?

[Simplex3]

I'm not ready to drop $200 on a captain's chair, but I'm past being able to comfortably do knee raises with my arms on the dip handles of my rack. I have a full cage, and I know I used to see straps that hook to the chin-up bar and then under your elbows, but I'll be damned if I can find them and I never liked them that much anyway, no control over your back.

I know there are some fitness buffs here, so wow me.