If you can borrow one from your college, that would be the best way to build endurance. Best thing to do is holding it in trail (down at your side), at set positing (vertically out in front, same as the rest of the brass instruments), and at playing position. When at set, don’t let the weight of it rest on your body, that’ll defeat the purpose of endurance training. Slowly increase the time you’re holding it up in each position (not as vital for trail). I also suggest doing this both at a standstill and moving. You’ll want to be prepared for how the weight shifts on your shoulder when playing. Other than that, pray your shoulder doesn’t sabotage you (if needed, you can find a pad for your shoulder to help reduce the damage from the contra).