The tub drain outlet is level with the drain line (where the drum trap outlet was).

Can I just make-up a "U" shaped trap?

You can take out the drum trap and put the "U" trap in the pipe where the drum trap was, but a better idea would be for you to put a "P" trap right on the end of the waste and overflow (this is the part that connects to the tub that has the overflow that goes up the tub and the shoe that goes under the tub) Then use PVC pipe to connect over to where you took out the drum trap out. You can make this connection with a "Fernco Adaptor", that is just sa brand name, it is a neoprene coupling that tightens with screw clamps. And yes the p-trap can be PVC also. Good luck.

The important thing is not to make an ''S'' trap. Where the drain makes it final turn to go down, it must be vented. If nothing else, you can add an AAV that lets the air in, but not the sewer gases out.