Sit-ups are ideally done on an incline bench, also called an abdominal bench,
so that the abs work harder than when you lay flat.
Rather than spend a bunch of money on a heavy metal bench, I simply made my own.
Seeking to make a stable bench,
I opted for using 2x8 planks, which are actually
7.25" wide by 1.5" thick.
I bought an 8-footer and got it cut into
two 4' pieces at the store for free.
Two 2x8 by 4 feet pieces.
Two 2x4 segments, each 14.5 inches wide.
Two 2x4 segments used to raise one end.
I used two 7.5 inch segments but it's a matter of preference.
Eighteen 3" drywall screws.
Total cost could be around $20.
Lay the 2x8's flat and screw on the stabilizing center piece, then the piece at the end.
Attach the two legs, being sure to add extra screws for strength.