1. Purchase a section of 1” diameter of cold rolled steel. If you want to approach the 300lbs mark, a 30 inch bar of steel will work. Most people don’t need that and 24 inches would work fine if you want to go around 200lbs. You can find hollow steel tubes at Home Depot but I’d rather have something solid, just for my piece of mind. I purchased mine from onlinemetals.com.
2. Purchase a bike grip, preferably for a BMX bike.
3. Purchase some 10, 5 and 2.5lb standard plates (not Olympic plates). I got mine at Play it Again Sports for very cheap. Buy whatever you think you will need to support your rowing habits.
4. Get some clamps for standard barbells. I use 2-3 standard sized spring clamps PER SIDE. You can never be too careful. You can also find clamps used for plumbing; the ease of getting the spring clamps on and off are great.