Get hold of an Eb cavalry trumpet like this one:
Gold Lacquer Cavalry Trumpet - Superb on eBay, also, Bugle, Brass, Musical Instruments (end time 22-Apr-08 00:51:28 BST)
Then organise some crooks.
Its much simpler than starting from scratch.
I used to have one, here is a picture:
I have also made my own natural trumpet from plumbing tubing and the bell off an old trumpet.
I did not have a spare leadpipe so I had to make one from interlocking sections of tubing off the donor instrument.
I finished it off with a tuneable leadpipe that I had lying around.
The secret was to make the instrument in two sections of tubing which could be gradually trimmed back to get the correct length.
I am on holiday and only have limited access to scanning etc but I am going to measure it all up and make photographs of it so you can have a go at making one.
I think it cost about £10 to make in total because I had the donor parts.
The best way to do the leadpipe is to use one from a Bb trumpet and adapt the slide end so it takes a clamp for tuning adjustment, then put the leg of the old tuning slide into the pipe it is attaching to.
I have seen this method used often and its very effective.