Well Sparky,
Here are two suggestions, one is a little more costly but less work, the other is more work, but since you have all the materials, might be a better route.
The first option would be to purchase a bike rack that slips into a 2" receiver. You could take a 6" x 4" plate and wield a piece of 2" ID (Inside diameter) to the plate, so that you have essentially made a receiver hitch. Drill a couple holes in the plate so you can attach it to the bottom, or top for that matter, of your bumper using a couple 4" U Bolts. This gives you a couple of options. One it can be easily removed from the hitch, but the hitc can be removed as well. I've found the bike racks in the classifieds in my area, or you can get them for around $100 at Walmart/Target/etc.... The beneift to this route is that the bike rack will already have the saddles or padding for the bikes and usually comes with built in straps. Finally, the bike racks you purchase usually will have a feature that allows them to collapse so someone doesn't run around the back of your TT and clothesline themselves. Here is a link to what I'm talking about at Target. (Hopefully it doesn't go against the rules since it's commercial?)
http://www.target.com/gp/detail.html/sr=1-8/qid=1178138021/ref=sr_1_8/602-9867526-9019857?ie=UTF8&asin=B000AYCJF0
The second option would be to use a 4x6" plate at the bottom, again, to be attached with U bots, so it can be removed. Weild to that a 3' peice of 3" square or round tubing. At the top you would weild a 2 - 2 1/2' cross bar to make a "T". ( you may want to measure the bikes you will be hauling so that the distance is not greater than the span) the distance above is an estimate. The from there you will want to extend more tubing (About the same distance 2-2 1/2' again depending on how many bikes you want to haul). Finally, you would want to put some padding of some sort so that the bikes don't get scratched up during transit. More simply put, you can look at the website above, and where it goes back into the trailer hitch, you would replace that with a plate you could mount to your bumper.
I hope this was at least helpful in getting you going in the right direction? Good luck.